<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Query Expressions — Django 1.8.19 documentation</title>
<link rel="stylesheet" href="../../_static/default.css" type="text/css" />
<link rel="stylesheet" href="../../_static/pygments.css" type="text/css" />
<script type="text/javascript">
var DOCUMENTATION_OPTIONS = {
URL_ROOT: '../../',
VERSION: '1.8.19',
COLLAPSE_INDEX: false,
FILE_SUFFIX: '.html',
HAS_SOURCE: true
};
</script>
<script type="text/javascript" src="../../_static/jquery.js"></script>
<script type="text/javascript" src="../../_static/underscore.js"></script>
<script type="text/javascript" src="../../_static/doctools.js"></script>
<link rel="index" title="Index" href="../../genindex.html" />
<link rel="search" title="Search" href="../../search.html" />
<link rel="top" title="Django 1.8.19 documentation" href="../../contents.html" />
<link rel="up" title="Models" href="index.html" />
<link rel="next" title="Conditional Expressions" href="conditional-expressions.html" />
<link rel="prev" title="Lookup API reference" href="lookups.html" />
<script type="text/javascript" src="../../templatebuiltins.js"></script>
<script type="text/javascript">
(function($) {
if (!django_template_builtins) {
// templatebuiltins.js missing, do nothing.
return;
}
$(document).ready(function() {
// Hyperlink Django template tags and filters
var base = "../templates/builtins.html";
if (base == "#") {
// Special case for builtins.html itself
base = "";
}
// Tags are keywords, class '.k'
$("div.highlight\\-html\\+django span.k").each(function(i, elem) {
var tagname = $(elem).text();
if ($.inArray(tagname, django_template_builtins.ttags) != -1) {
var fragment = tagname.replace(/_/, '-');
$(elem).html("<a href='" + base + "#" + fragment + "'>" + tagname + "</a>");
}
});
// Filters are functions, class '.nf'
$("div.highlight\\-html\\+django span.nf").each(function(i, elem) {
var filtername = $(elem).text();
if ($.inArray(filtername, django_template_builtins.tfilters) != -1) {
var fragment = filtername.replace(/_/, '-');
$(elem).html("<a href='" + base + "#" + fragment + "'>" + filtername + "</a>");
}
});
});
})(jQuery);
</script>
</head>
<body role="document">
<div class="document">
<div id="custom-doc" class="yui-t6">
<div id="hd">
<h1><a href="../../index.html">Django 1.8.19 documentation</a></h1>
<div id="global-nav">
<a title="Home page" href="../../index.html">Home</a> |
<a title="Table of contents" href="../../contents.html">Table of contents</a> |
<a title="Global index" href="../../genindex.html">Index</a> |
<a title="Module index" href="../../py-modindex.html">Modules</a>
</div>
<div class="nav">
« <a href="lookups.html" title="Lookup API reference">previous</a>
|
<a href="../index.html" title="API Reference" accesskey="U">up</a>
|
<a href="conditional-expressions.html" title="Conditional Expressions">next</a> »</div>
</div>
<div id="bd">
<div id="yui-main">
<div class="yui-b">
<div class="yui-g" id="ref-models-expressions">
<div class="section" id="s-query-expressions">
<span id="query-expressions"></span><h1>Query Expressions<a class="headerlink" href="#query-expressions" title="Permalink to this headline">¶</a></h1>
<p>Query expressions describe a value or a computation that can be used as part of
a filter, order by, annotation, or aggregate. There are a number of built-in
expressions (documented below) that can be used to help you write queries.
Expressions can be combined, or in some cases nested, to form more complex
computations.</p>
<div class="section" id="s-supported-arithmetic">
<span id="supported-arithmetic"></span><h2>Supported arithmetic<a class="headerlink" href="#supported-arithmetic" title="Permalink to this headline">¶</a></h2>
<p>Django supports addition, subtraction, multiplication, division, modulo
arithmetic, and the power operator on query expressions, using Python constants,
variables, and even other expressions.</p>
<div class="versionadded">
<span class="title">New in Django 1.7:</span> <p>Support for the power operator <code class="docutils literal"><span class="pre">**</span></code> was added.</p>
</div>
</div>
<div class="section" id="s-some-examples">
<span id="some-examples"></span><h2>Some examples<a class="headerlink" href="#some-examples" title="Permalink to this headline">¶</a></h2>
<div class="versionchanged">
<span class="title">Changed in Django 1.8:</span> <p>Some of the examples rely on functionality that is new in Django 1.8.</p>
</div>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="kn">import</span> <span class="n">F</span><span class="p">,</span> <span class="n">Count</span>
<span class="kn">from</span> <span class="nn">django.db.models.functions</span> <span class="kn">import</span> <span class="n">Length</span>
<span class="c1"># Find companies that have more employees than chairs.</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">num_employees__gt</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'num_chairs'</span><span class="p">))</span>
<span class="c1"># Find companies that have at least twice as many employees</span>
<span class="c1"># as chairs. Both the querysets below are equivalent.</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">num_employees__gt</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'num_chairs'</span><span class="p">)</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span>
<span class="n">num_employees__gt</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'num_chairs'</span><span class="p">)</span> <span class="o">+</span> <span class="n">F</span><span class="p">(</span><span class="s1">'num_chairs'</span><span class="p">))</span>
<span class="c1"># How many chairs are needed for each company to seat all employees?</span>
<span class="o">>>></span> <span class="n">company</span> <span class="o">=</span> <span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span>
<span class="o">...</span> <span class="n">num_employees__gt</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'num_chairs'</span><span class="p">))</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span>
<span class="o">...</span> <span class="n">chairs_needed</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'num_employees'</span><span class="p">)</span> <span class="o">-</span> <span class="n">F</span><span class="p">(</span><span class="s1">'num_chairs'</span><span class="p">))</span><span class="o">.</span><span class="n">first</span><span class="p">()</span>
<span class="o">>>></span> <span class="n">company</span><span class="o">.</span><span class="n">num_employees</span>
<span class="mi">120</span>
<span class="o">>>></span> <span class="n">company</span><span class="o">.</span><span class="n">num_chairs</span>
<span class="mi">50</span>
<span class="o">>>></span> <span class="n">company</span><span class="o">.</span><span class="n">chairs_needed</span>
<span class="mi">70</span>
<span class="c1"># Annotate models with an aggregated value. Both forms</span>
<span class="c1"># below are equivalent.</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">num_products</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="s1">'products'</span><span class="p">))</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">num_products</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="n">F</span><span class="p">(</span><span class="s1">'products'</span><span class="p">)))</span>
<span class="c1"># Aggregates can contain complex computations also</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">num_offerings</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="n">F</span><span class="p">(</span><span class="s1">'products'</span><span class="p">)</span> <span class="o">+</span> <span class="n">F</span><span class="p">(</span><span class="s1">'services'</span><span class="p">)))</span>
<span class="c1"># Expressions can also be used in order_by()</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="n">Length</span><span class="p">(</span><span class="s1">'name'</span><span class="p">)</span><span class="o">.</span><span class="n">asc</span><span class="p">())</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="n">Length</span><span class="p">(</span><span class="s1">'name'</span><span class="p">)</span><span class="o">.</span><span class="n">desc</span><span class="p">())</span>
</pre></div>
</div>
</div>
<div class="section" id="s-built-in-expressions">
<span id="built-in-expressions"></span><h2>Built-in Expressions<a class="headerlink" href="#built-in-expressions" title="Permalink to this headline">¶</a></h2>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">These expressions are defined in <code class="docutils literal"><span class="pre">django.db.models.expressions</span></code> and
<code class="docutils literal"><span class="pre">django.db.models.aggregates</span></code>, but for convenience they’re available and
usually imported from <a class="reference internal" href="../../topics/db/models.html#module-django.db.models" title="django.db.models"><code class="xref py py-mod docutils literal"><span class="pre">django.db.models</span></code></a>.</p>
</div>
<div class="section" id="s-f-expressions">
<span id="f-expressions"></span><h3><code class="docutils literal"><span class="pre">F()</span></code> expressions<a class="headerlink" href="#f-expressions" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="django.db.models.F">
<em class="property">class </em><code class="descname">F</code><a class="reference internal" href="../../_modules/django/db/models/expressions.html#F"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.F" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>An <code class="docutils literal"><span class="pre">F()</span></code> object represents the value of a model field or annotated column. It
makes it possible to refer to model field values and perform database
operations using them without actually having to pull them out of the database
into Python memory.</p>
<p>Instead, Django uses the <code class="docutils literal"><span class="pre">F()</span></code> object to generate a SQL expression that
describes the required operation at the database level.</p>
<p>This is easiest to understand through an example. Normally, one might do
something like this:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># Tintin filed a news story!</span>
<span class="n">reporter</span> <span class="o">=</span> <span class="n">Reporters</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Tintin'</span><span class="p">)</span>
<span class="n">reporter</span><span class="o">.</span><span class="n">stories_filed</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="n">reporter</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
</pre></div>
</div>
<p>Here, we have pulled the value of <code class="docutils literal"><span class="pre">reporter.stories_filed</span></code> from the database
into memory and manipulated it using familiar Python operators, and then saved
the object back to the database. But instead we could also have done:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">F</span>
<span class="n">reporter</span> <span class="o">=</span> <span class="n">Reporters</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Tintin'</span><span class="p">)</span>
<span class="n">reporter</span><span class="o">.</span><span class="n">stories_filed</span> <span class="o">=</span> <span class="n">F</span><span class="p">(</span><span class="s1">'stories_filed'</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span>
<span class="n">reporter</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
</pre></div>
</div>
<p>Although <code class="docutils literal"><span class="pre">reporter.stories_filed</span> <span class="pre">=</span> <span class="pre">F('stories_filed')</span> <span class="pre">+</span> <span class="pre">1</span></code> looks like a
normal Python assignment of value to an instance attribute, in fact it’s an SQL
construct describing an operation on the database.</p>
<p>When Django encounters an instance of <code class="docutils literal"><span class="pre">F()</span></code>, it overrides the standard Python
operators to create an encapsulated SQL expression; in this case, one which
instructs the database to increment the database field represented by
<code class="docutils literal"><span class="pre">reporter.stories_filed</span></code>.</p>
<p>Whatever value is or was on <code class="docutils literal"><span class="pre">reporter.stories_filed</span></code>, Python never gets to
know about it - it is dealt with entirely by the database. All Python does,
through Django’s <code class="docutils literal"><span class="pre">F()</span></code> class, is create the SQL syntax to refer to the field
and describe the operation.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>In order to access the new value that has been saved in this way, the object
will need to be reloaded:</p>
<div class="last highlight-default"><div class="highlight"><pre><span></span><span class="n">reporter</span> <span class="o">=</span> <span class="n">Reporters</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">pk</span><span class="o">=</span><span class="n">reporter</span><span class="o">.</span><span class="n">pk</span><span class="p">)</span>
</pre></div>
</div>
</div>
<p>As well as being used in operations on single instances as above, <code class="docutils literal"><span class="pre">F()</span></code> can
be used on <code class="docutils literal"><span class="pre">QuerySets</span></code> of object instances, with <code class="docutils literal"><span class="pre">update()</span></code>. This reduces
the two queries we were using above - the <code class="docutils literal"><span class="pre">get()</span></code> and the
<a class="reference internal" href="instances.html#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal"><span class="pre">save()</span></code></a> - to just one:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">reporter</span> <span class="o">=</span> <span class="n">Reporters</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Tintin'</span><span class="p">)</span>
<span class="n">reporter</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">stories_filed</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'stories_filed'</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
<p>We can also use <a class="reference internal" href="querysets.html#django.db.models.query.QuerySet.update" title="django.db.models.query.QuerySet.update"><code class="xref py py-meth docutils literal"><span class="pre">update()</span></code></a> to increment
the field value on multiple objects - which could be very much faster than
pulling them all into Python from the database, looping over them, incrementing
the field value of each one, and saving each one back to the database:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">Reporter</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">all</span><span class="p">()</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">stories_filed</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'stories_filed'</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">F()</span></code> therefore can offer performance advantages by:</p>
<ul class="simple">
<li>getting the database, rather than Python, to do work</li>
<li>reducing the number of queries some operations require</li>
</ul>
<div class="section" id="s-avoiding-race-conditions-using-f">
<span id="s-id1"></span><span id="avoiding-race-conditions-using-f"></span><span id="id1"></span><h4>Avoiding race conditions using <code class="docutils literal"><span class="pre">F()</span></code><a class="headerlink" href="#avoiding-race-conditions-using-f" title="Permalink to this headline">¶</a></h4>
<p>Another useful benefit of <code class="docutils literal"><span class="pre">F()</span></code> is that having the database - rather than
Python - update a field’s value avoids a <em>race condition</em>.</p>
<p>If two Python threads execute the code in the first example above, one thread
could retrieve, increment, and save a field’s value after the other has
retrieved it from the database. The value that the second thread saves will be
based on the original value; the work of the first thread will simply be lost.</p>
<p>If the database is responsible for updating the field, the process is more
robust: it will only ever update the field based on the value of the field in
the database when the <a class="reference internal" href="instances.html#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal"><span class="pre">save()</span></code></a> or <code class="docutils literal"><span class="pre">update()</span></code> is executed, rather
than based on its value when the instance was retrieved.</p>
</div>
<div class="section" id="s-using-f-in-filters">
<span id="using-f-in-filters"></span><h4>Using <code class="docutils literal"><span class="pre">F()</span></code> in filters<a class="headerlink" href="#using-f-in-filters" title="Permalink to this headline">¶</a></h4>
<p><code class="docutils literal"><span class="pre">F()</span></code> is also very useful in <code class="docutils literal"><span class="pre">QuerySet</span></code> filters, where they make it
possible to filter a set of objects against criteria based on their field
values, rather than on Python values.</p>
<p>This is documented in <a class="reference internal" href="../../topics/db/queries.html#using-f-expressions-in-filters"><span class="std std-ref">using F() expressions in queries</span></a>.</p>
</div>
<div class="section" id="s-using-f-with-annotations">
<span id="s-id2"></span><span id="using-f-with-annotations"></span><span id="id2"></span><h4>Using <code class="docutils literal"><span class="pre">F()</span></code> with annotations<a class="headerlink" href="#using-f-with-annotations" title="Permalink to this headline">¶</a></h4>
<p><code class="docutils literal"><span class="pre">F()</span></code> can be used to create dynamic fields on your models by combining
different fields with arithmetic:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">company</span> <span class="o">=</span> <span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span>
<span class="n">chairs_needed</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'num_employees'</span><span class="p">)</span> <span class="o">-</span> <span class="n">F</span><span class="p">(</span><span class="s1">'num_chairs'</span><span class="p">))</span>
</pre></div>
</div>
<p>If the fields that you’re combining are of different types you’ll need
to tell Django what kind of field will be returned. Since <code class="docutils literal"><span class="pre">F()</span></code> does not
directly support <code class="docutils literal"><span class="pre">output_field</span></code> you will need to wrap the expression with
<a class="reference internal" href="#django.db.models.ExpressionWrapper" title="django.db.models.ExpressionWrapper"><code class="xref py py-class docutils literal"><span class="pre">ExpressionWrapper</span></code></a>:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">DateTimeField</span><span class="p">,</span> <span class="n">ExpressionWrapper</span><span class="p">,</span> <span class="n">F</span>
<span class="n">Ticket</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span>
<span class="n">expires</span><span class="o">=</span><span class="n">ExpressionWrapper</span><span class="p">(</span>
<span class="n">F</span><span class="p">(</span><span class="s1">'active_at'</span><span class="p">)</span> <span class="o">+</span> <span class="n">F</span><span class="p">(</span><span class="s1">'duration'</span><span class="p">),</span> <span class="n">output_field</span><span class="o">=</span><span class="n">DateTimeField</span><span class="p">()))</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="s-func-expressions">
<span id="s-id3"></span><span id="func-expressions"></span><span id="id3"></span><h3><code class="docutils literal"><span class="pre">Func()</span></code> expressions<a class="headerlink" href="#func-expressions" title="Permalink to this headline">¶</a></h3>
<div class="versionadded">
<span class="title">New in Django 1.8.</span> </div>
<p><code class="docutils literal"><span class="pre">Func()</span></code> expressions are the base type of all expressions that involve
database functions like <code class="docutils literal"><span class="pre">COALESCE</span></code> and <code class="docutils literal"><span class="pre">LOWER</span></code>, or aggregates like <code class="docutils literal"><span class="pre">SUM</span></code>.
They can be used directly:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">Func</span><span class="p">,</span> <span class="n">F</span>
<span class="n">queryset</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">field_lower</span><span class="o">=</span><span class="n">Func</span><span class="p">(</span><span class="n">F</span><span class="p">(</span><span class="s1">'field'</span><span class="p">),</span> <span class="n">function</span><span class="o">=</span><span class="s1">'LOWER'</span><span class="p">))</span>
</pre></div>
</div>
<p>or they can be used to build a library of database functions:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Lower</span><span class="p">(</span><span class="n">Func</span><span class="p">):</span>
<span class="n">function</span> <span class="o">=</span> <span class="s1">'LOWER'</span>
<span class="n">queryset</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">field_lower</span><span class="o">=</span><span class="n">Lower</span><span class="p">(</span><span class="s1">'field'</span><span class="p">))</span>
</pre></div>
</div>
<p>But both cases will result in a queryset where each model is annotated with an
extra attribute <code class="docutils literal"><span class="pre">field_lower</span></code> produced, roughly, from the following SQL:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">SELECT</span>
<span class="o">...</span>
<span class="n">LOWER</span><span class="p">(</span><span class="s2">"db_table"</span><span class="o">.</span><span class="s2">"field"</span><span class="p">)</span> <span class="k">as</span> <span class="s2">"field_lower"</span>
</pre></div>
</div>
<p>See <a class="reference internal" href="database-functions.html"><span class="doc">Database Functions</span></a> for a list of built-in database functions.</p>
<p>The <code class="docutils literal"><span class="pre">Func</span></code> API is as follows:</p>
<dl class="class">
<dt id="django.db.models.Func">
<em class="property">class </em><code class="descname">Func</code>(<em>*expressions</em>, <em>**extra</em>)<a class="reference internal" href="../../_modules/django/db/models/expressions.html#Func"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Func" title="Permalink to this definition">¶</a></dt>
<dd><dl class="attribute">
<dt id="django.db.models.Func.function">
<code class="descname">function</code><a class="headerlink" href="#django.db.models.Func.function" title="Permalink to this definition">¶</a></dt>
<dd><p>A class attribute describing the function that will be generated.
Specifically, the <code class="docutils literal"><span class="pre">function</span></code> will be interpolated as the <code class="docutils literal"><span class="pre">function</span></code>
placeholder within <a class="reference internal" href="#django.db.models.Func.template" title="django.db.models.Func.template"><code class="xref py py-attr docutils literal"><span class="pre">template</span></code></a>. Defaults to <code class="docutils literal"><span class="pre">None</span></code>.</p>
</dd></dl>
<dl class="attribute">
<dt id="django.db.models.Func.template">
<code class="descname">template</code><a class="headerlink" href="#django.db.models.Func.template" title="Permalink to this definition">¶</a></dt>
<dd><p>A class attribute, as a format string, that describes the SQL that is
generated for this function. Defaults to
<code class="docutils literal"><span class="pre">'%(function)s(%(expressions)s)'</span></code>.</p>
</dd></dl>
<dl class="attribute">
<dt id="django.db.models.Func.arg_joiner">
<code class="descname">arg_joiner</code><a class="headerlink" href="#django.db.models.Func.arg_joiner" title="Permalink to this definition">¶</a></dt>
<dd><p>A class attribute that denotes the character used to join the list of
<code class="docutils literal"><span class="pre">expressions</span></code> together. Defaults to <code class="docutils literal"><span class="pre">',</span> <span class="pre">'</span></code>.</p>
</dd></dl>
</dd></dl>
<p>The <code class="docutils literal"><span class="pre">*expressions</span></code> argument is a list of positional expressions that the
function will be applied to. The expressions will be converted to strings,
joined together with <code class="docutils literal"><span class="pre">arg_joiner</span></code>, and then interpolated into the <code class="docutils literal"><span class="pre">template</span></code>
as the <code class="docutils literal"><span class="pre">expressions</span></code> placeholder.</p>
<p>Positional arguments can be expressions or Python values. Strings are
assumed to be column references and will be wrapped in <code class="docutils literal"><span class="pre">F()</span></code> expressions
while other values will be wrapped in <code class="docutils literal"><span class="pre">Value()</span></code> expressions.</p>
<p>The <code class="docutils literal"><span class="pre">**extra</span></code> kwargs are <code class="docutils literal"><span class="pre">key=value</span></code> pairs that can be interpolated
into the <code class="docutils literal"><span class="pre">template</span></code> attribute. Note that the keywords <code class="docutils literal"><span class="pre">function</span></code> and
<code class="docutils literal"><span class="pre">template</span></code> can be used to replace the <code class="docutils literal"><span class="pre">function</span></code> and <code class="docutils literal"><span class="pre">template</span></code>
attributes respectively, without having to define your own class.
<code class="docutils literal"><span class="pre">output_field</span></code> can be used to define the expected return type.</p>
</div>
<div class="section" id="s-aggregate-expressions">
<span id="aggregate-expressions"></span><h3><code class="docutils literal"><span class="pre">Aggregate()</span></code> expressions<a class="headerlink" href="#aggregate-expressions" title="Permalink to this headline">¶</a></h3>
<p>An aggregate expression is a special case of a <a class="reference internal" href="#func-expressions"><span class="std std-ref">Func() expression</span></a> that informs the query that a <code class="docutils literal"><span class="pre">GROUP</span> <span class="pre">BY</span></code> clause
is required. All of the <a class="reference internal" href="querysets.html#aggregation-functions"><span class="std std-ref">aggregate functions</span></a>,
like <code class="docutils literal"><span class="pre">Sum()</span></code> and <code class="docutils literal"><span class="pre">Count()</span></code>, inherit from <code class="docutils literal"><span class="pre">Aggregate()</span></code>.</p>
<p>Since <code class="docutils literal"><span class="pre">Aggregate</span></code>s are expressions and wrap expressions, you can represent
some complex computations:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">Count</span>
<span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span>
<span class="n">managers_required</span><span class="o">=</span><span class="p">(</span><span class="n">Count</span><span class="p">(</span><span class="s1">'num_employees'</span><span class="p">)</span> <span class="o">/</span> <span class="mi">4</span><span class="p">)</span> <span class="o">+</span> <span class="n">Count</span><span class="p">(</span><span class="s1">'num_managers'</span><span class="p">))</span>
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">Aggregate</span></code> API is as follows:</p>
<dl class="class">
<dt id="django.db.models.Aggregate">
<em class="property">class </em><code class="descname">Aggregate</code>(<em>expression</em>, <em>output_field=None</em>, <em>**extra</em>)<a class="reference internal" href="../../_modules/django/db/models/aggregates.html#Aggregate"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Aggregate" title="Permalink to this definition">¶</a></dt>
<dd><dl class="attribute">
<dt id="django.db.models.Aggregate.template">
<code class="descname">template</code><a class="headerlink" href="#django.db.models.Aggregate.template" title="Permalink to this definition">¶</a></dt>
<dd><p>A class attribute, as a format string, that describes the SQL that is
generated for this aggregate. Defaults to
<code class="docutils literal"><span class="pre">'%(function)s(</span> <span class="pre">%(expressions)s</span> <span class="pre">)'</span></code>.</p>
</dd></dl>
<dl class="attribute">
<dt id="django.db.models.Aggregate.function">
<code class="descname">function</code><a class="headerlink" href="#django.db.models.Aggregate.function" title="Permalink to this definition">¶</a></dt>
<dd><p>A class attribute describing the aggregate function that will be
generated. Specifically, the <code class="docutils literal"><span class="pre">function</span></code> will be interpolated as the
<code class="docutils literal"><span class="pre">function</span></code> placeholder within <a class="reference internal" href="#django.db.models.Aggregate.template" title="django.db.models.Aggregate.template"><code class="xref py py-attr docutils literal"><span class="pre">template</span></code></a>. Defaults to <code class="docutils literal"><span class="pre">None</span></code>.</p>
</dd></dl>
</dd></dl>
<p>The <code class="docutils literal"><span class="pre">expression</span></code> argument can be the name of a field on the model, or another
expression. It will be converted to a string and used as the <code class="docutils literal"><span class="pre">expressions</span></code>
placeholder within the <code class="docutils literal"><span class="pre">template</span></code>.</p>
<p>The <code class="docutils literal"><span class="pre">output_field</span></code> argument requires a model field instance, like
<code class="docutils literal"><span class="pre">IntegerField()</span></code> or <code class="docutils literal"><span class="pre">BooleanField()</span></code>, into which Django will load the value
after it’s retrieved from the database. Usually no arguments are needed when
instantiating the model field as any arguments relating to data validation
(<code class="docutils literal"><span class="pre">max_length</span></code>, <code class="docutils literal"><span class="pre">max_digits</span></code>, etc.) will not be enforced on the expression’s
output value.</p>
<p>Note that <code class="docutils literal"><span class="pre">output_field</span></code> is only required when Django is unable to determine
what field type the result should be. Complex expressions that mix field types
should define the desired <code class="docutils literal"><span class="pre">output_field</span></code>. For example, adding an
<code class="docutils literal"><span class="pre">IntegerField()</span></code> and a <code class="docutils literal"><span class="pre">FloatField()</span></code> together should probably have
<code class="docutils literal"><span class="pre">output_field=FloatField()</span></code> defined.</p>
<div class="versionchanged">
<span class="title">Changed in Django 1.8:</span> <p><code class="docutils literal"><span class="pre">output_field</span></code> is a new parameter.</p>
</div>
<p>The <code class="docutils literal"><span class="pre">**extra</span></code> kwargs are <code class="docutils literal"><span class="pre">key=value</span></code> pairs that can be interpolated
into the <code class="docutils literal"><span class="pre">template</span></code> attribute.</p>
<div class="versionadded">
<span class="title">New in Django 1.8:</span> <p>Aggregate functions can now use arithmetic and reference multiple
model fields in a single function.</p>
</div>
</div>
<div class="section" id="s-creating-your-own-aggregate-functions">
<span id="creating-your-own-aggregate-functions"></span><h3>Creating your own Aggregate Functions<a class="headerlink" href="#creating-your-own-aggregate-functions" title="Permalink to this headline">¶</a></h3>
<p>Creating your own aggregate is extremely easy. At a minimum, you need
to define <code class="docutils literal"><span class="pre">function</span></code>, but you can also completely customize the
SQL that is generated. Here’s a brief example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">Aggregate</span>
<span class="k">class</span> <span class="nc">Count</span><span class="p">(</span><span class="n">Aggregate</span><span class="p">):</span>
<span class="c1"># supports COUNT(distinct field)</span>
<span class="n">function</span> <span class="o">=</span> <span class="s1">'COUNT'</span>
<span class="n">template</span> <span class="o">=</span> <span class="s1">'</span><span class="si">%(function)s</span><span class="s1">(</span><span class="si">%(distinct)s%(expressions)s</span><span class="s1">)'</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">expression</span><span class="p">,</span> <span class="n">distinct</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="o">**</span><span class="n">extra</span><span class="p">):</span>
<span class="nb">super</span><span class="p">(</span><span class="n">Count</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span>
<span class="n">expression</span><span class="p">,</span>
<span class="n">distinct</span><span class="o">=</span><span class="s1">'DISTINCT '</span> <span class="k">if</span> <span class="n">distinct</span> <span class="k">else</span> <span class="s1">''</span><span class="p">,</span>
<span class="n">output_field</span><span class="o">=</span><span class="n">IntegerField</span><span class="p">(),</span>
<span class="o">**</span><span class="n">extra</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="s-value-expressions">
<span id="value-expressions"></span><h3><code class="docutils literal"><span class="pre">Value()</span></code> expressions<a class="headerlink" href="#value-expressions" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="django.db.models.Value">
<em class="property">class </em><code class="descname">Value</code>(<em>value</em>, <em>output_field=None</em>)<a class="reference internal" href="../../_modules/django/db/models/expressions.html#Value"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Value" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>A <code class="docutils literal"><span class="pre">Value()</span></code> object represents the smallest possible component of an
expression: a simple value. When you need to represent the value of an integer,
boolean, or string within an expression, you can wrap that value within a
<code class="docutils literal"><span class="pre">Value()</span></code>.</p>
<p>You will rarely need to use <code class="docutils literal"><span class="pre">Value()</span></code> directly. When you write the expression
<code class="docutils literal"><span class="pre">F('field')</span> <span class="pre">+</span> <span class="pre">1</span></code>, Django implicitly wraps the <code class="docutils literal"><span class="pre">1</span></code> in a <code class="docutils literal"><span class="pre">Value()</span></code>,
allowing simple values to be used in more complex expressions.</p>
<p>The <code class="docutils literal"><span class="pre">value</span></code> argument describes the value to be included in the expression,
such as <code class="docutils literal"><span class="pre">1</span></code>, <code class="docutils literal"><span class="pre">True</span></code>, or <code class="docutils literal"><span class="pre">None</span></code>. Django knows how to convert these Python
values into their corresponding database type.</p>
<p>The <code class="docutils literal"><span class="pre">output_field</span></code> argument should be a model field instance, like
<code class="docutils literal"><span class="pre">IntegerField()</span></code> or <code class="docutils literal"><span class="pre">BooleanField()</span></code>, into which Django will load the value
after it’s retrieved from the database. Usually no arguments are needed when
instantiating the model field as any arguments relating to data validation
(<code class="docutils literal"><span class="pre">max_length</span></code>, <code class="docutils literal"><span class="pre">max_digits</span></code>, etc.) will not be enforced on the expression’s
output value.</p>
</div>
<div class="section" id="s-expressionwrapper-expressions">
<span id="expressionwrapper-expressions"></span><h3><code class="docutils literal"><span class="pre">ExpressionWrapper()</span></code> expressions<a class="headerlink" href="#expressionwrapper-expressions" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="django.db.models.ExpressionWrapper">
<em class="property">class </em><code class="descname">ExpressionWrapper</code>(<em>expression</em>, <em>output_field</em>)<a class="reference internal" href="../../_modules/django/db/models/expressions.html#ExpressionWrapper"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.ExpressionWrapper" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<div class="versionadded">
<span class="title">New in Django 1.8.</span> </div>
<p><code class="docutils literal"><span class="pre">ExpressionWrapper</span></code> simply surrounds another expression and provides access
to properties, such as <code class="docutils literal"><span class="pre">output_field</span></code>, that may not be available on other
expressions. <code class="docutils literal"><span class="pre">ExpressionWrapper</span></code> is necessary when using arithmetic on
<code class="docutils literal"><span class="pre">F()</span></code> expressions with different types as described in
<a class="reference internal" href="#using-f-with-annotations"><span class="std std-ref">Using F() with annotations</span></a>.</p>
</div>
<div class="section" id="s-conditional-expressions">
<span id="conditional-expressions"></span><h3>Conditional expressions<a class="headerlink" href="#conditional-expressions" title="Permalink to this headline">¶</a></h3>
<div class="versionadded">
<span class="title">New in Django 1.8.</span> </div>
<p>Conditional expressions allow you to use <code class="xref std std-keyword docutils literal"><span class="pre">if</span></code> ... <code class="xref std std-keyword docutils literal"><span class="pre">elif</span></code> ...
<code class="xref std std-keyword docutils literal"><span class="pre">else</span></code> logic in queries. Django natively supports SQL <code class="docutils literal"><span class="pre">CASE</span></code>
expressions. For more details see <a class="reference internal" href="conditional-expressions.html"><span class="doc">Conditional Expressions</span></a>.</p>
</div>
<div class="section" id="s-raw-sql-expressions">
<span id="raw-sql-expressions"></span><h3>Raw SQL expressions<a class="headerlink" href="#raw-sql-expressions" title="Permalink to this headline">¶</a></h3>
<div class="versionadded">
<span class="title">New in Django 1.8.</span> </div>
<dl class="class">
<dt id="django.db.models.expressions.RawSQL">
<em class="property">class </em><code class="descname">RawSQL</code>(<em>sql</em>, <em>params</em>, <em>output_field=None</em>)<a class="reference internal" href="../../_modules/django/db/models/expressions.html#RawSQL"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.expressions.RawSQL" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>Sometimes database expressions can’t easily express a complex <code class="docutils literal"><span class="pre">WHERE</span></code> clause.
In these edge cases, use the <code class="docutils literal"><span class="pre">RawSQL</span></code> expression. For example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">django.db.models.expressions</span> <span class="k">import</span> <span class="n">RawSQL</span>
<span class="gp">>>> </span><span class="n">queryset</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">val</span><span class="o">=</span><span class="n">RawSQL</span><span class="p">(</span><span class="s2">"select col from sometable where othercol = </span><span class="si">%s</span><span class="s2">"</span><span class="p">,</span> <span class="p">(</span><span class="n">someparam</span><span class="p">,)))</span>
</pre></div>
</div>
<p>These extra lookups may not be portable to different database engines (because
you’re explicitly writing SQL code) and violate the DRY principle, so you
should avoid them if possible.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">You should be very careful to escape any parameters that the user can
control by using <code class="docutils literal"><span class="pre">params</span></code> in order to protect against <a class="reference internal" href="../../topics/security.html#sql-injection-protection"><span class="std std-ref">SQL injection
attacks</span></a>.</p>
</div>
</div>
</div>
<div class="section" id="s-technical-information">
<span id="technical-information"></span><h2>Technical Information<a class="headerlink" href="#technical-information" title="Permalink to this headline">¶</a></h2>
<p>Below you’ll find technical implementation details that may be useful to
library authors. The technical API and examples below will help with
creating generic query expressions that can extend the built-in functionality
that Django provides.</p>
<div class="section" id="s-expression-api">
<span id="expression-api"></span><h3>Expression API<a class="headerlink" href="#expression-api" title="Permalink to this headline">¶</a></h3>
<p>Query expressions implement the <a class="reference internal" href="lookups.html#query-expression"><span class="std std-ref">query expression API</span></a>,
but also expose a number of extra methods and attributes listed below. All
query expressions must inherit from <code class="docutils literal"><span class="pre">Expression()</span></code> or a relevant
subclass.</p>
<p>When a query expression wraps another expression, it is responsible for
calling the appropriate methods on the wrapped expression.</p>
<dl class="class">
<dt id="django.db.models.Expression">
<em class="property">class </em><code class="descname">Expression</code><a class="reference internal" href="../../_modules/django/db/models/expressions.html#Expression"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Expression" title="Permalink to this definition">¶</a></dt>
<dd><dl class="attribute">
<dt id="django.db.models.Expression.contains_aggregate">
<code class="descname">contains_aggregate</code><a class="headerlink" href="#django.db.models.Expression.contains_aggregate" title="Permalink to this definition">¶</a></dt>
<dd><p>Tells Django that this expression contains an aggregate and that a
<code class="docutils literal"><span class="pre">GROUP</span> <span class="pre">BY</span></code> clause needs to be added to the query.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.resolve_expression">
<code class="descname">resolve_expression</code>(<em>query=None</em>, <em>allow_joins=True</em>, <em>reuse=None</em>, <em>summarize=False</em>, <em>for_save=False</em>)<a class="headerlink" href="#django.db.models.Expression.resolve_expression" title="Permalink to this definition">¶</a></dt>
<dd><p>Provides the chance to do any pre-processing or validation of
the expression before it’s added to the query. <code class="docutils literal"><span class="pre">resolve_expression()</span></code>
must also be called on any nested expressions. A <code class="docutils literal"><span class="pre">copy()</span></code> of <code class="docutils literal"><span class="pre">self</span></code>
should be returned with any necessary transformations.</p>
<p><code class="docutils literal"><span class="pre">query</span></code> is the backend query implementation.</p>
<p><code class="docutils literal"><span class="pre">allow_joins</span></code> is a boolean that allows or denies the use of
joins in the query.</p>
<p><code class="docutils literal"><span class="pre">reuse</span></code> is a set of reusable joins for multi-join scenarios.</p>
<p><code class="docutils literal"><span class="pre">summarize</span></code> is a boolean that, when <code class="docutils literal"><span class="pre">True</span></code>, signals that the
query being computed is a terminal aggregate query.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.get_source_expressions">
<code class="descname">get_source_expressions</code>()<a class="headerlink" href="#django.db.models.Expression.get_source_expressions" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns an ordered list of inner expressions. For example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">Sum</span><span class="p">(</span><span class="n">F</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">))</span><span class="o">.</span><span class="n">get_source_expressions</span><span class="p">()</span>
<span class="go">[F('foo')]</span>
</pre></div>
</div>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.set_source_expressions">
<code class="descname">set_source_expressions</code>(<em>expressions</em>)<a class="headerlink" href="#django.db.models.Expression.set_source_expressions" title="Permalink to this definition">¶</a></dt>
<dd><p>Takes a list of expressions and stores them such that
<code class="docutils literal"><span class="pre">get_source_expressions()</span></code> can return them.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.relabeled_clone">
<code class="descname">relabeled_clone</code>(<em>change_map</em>)<a class="headerlink" href="#django.db.models.Expression.relabeled_clone" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns a clone (copy) of <code class="docutils literal"><span class="pre">self</span></code>, with any column aliases relabeled.
Column aliases are renamed when subqueries are created.
<code class="docutils literal"><span class="pre">relabeled_clone()</span></code> should also be called on any nested expressions
and assigned to the clone.</p>
<p><code class="docutils literal"><span class="pre">change_map</span></code> is a dictionary mapping old aliases to new aliases.</p>
<p>Example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">relabeled_clone</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">change_map</span><span class="p">):</span>
<span class="n">clone</span> <span class="o">=</span> <span class="n">copy</span><span class="o">.</span><span class="n">copy</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="n">clone</span><span class="o">.</span><span class="n">expression</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">expression</span><span class="o">.</span><span class="n">relabeled_clone</span><span class="p">(</span><span class="n">change_map</span><span class="p">)</span>
<span class="k">return</span> <span class="n">clone</span>
</pre></div>
</div>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.convert_value">
<code class="descname">convert_value</code>(<em>self</em>, <em>value</em>, <em>expression</em>, <em>connection</em>, <em>context</em>)<a class="headerlink" href="#django.db.models.Expression.convert_value" title="Permalink to this definition">¶</a></dt>
<dd><p>A hook allowing the expression to coerce <code class="docutils literal"><span class="pre">value</span></code> into a more
appropriate type.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.refs_aggregate">
<code class="descname">refs_aggregate</code>(<em>existing_aggregates</em>)<a class="headerlink" href="#django.db.models.Expression.refs_aggregate" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns a tuple containing the <code class="docutils literal"><span class="pre">(aggregate,</span> <span class="pre">lookup_path)</span></code> of the
first aggregate that this expression (or any nested expression)
references, or <code class="docutils literal"><span class="pre">(False,</span> <span class="pre">())</span></code> if no aggregate is referenced.
For example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">queryset</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">num_chairs__gt</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'sum__employees'</span><span class="p">))</span>
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">F()</span></code> expression here references a previous <code class="docutils literal"><span class="pre">Sum()</span></code>
computation which means that this filter expression should be
added to the <code class="docutils literal"><span class="pre">HAVING</span></code> clause rather than the <code class="docutils literal"><span class="pre">WHERE</span></code> clause.</p>
<p>In the majority of cases, returning the result of <code class="docutils literal"><span class="pre">refs_aggregate</span></code>
on any nested expression should be appropriate, as the necessary
built-in expressions will return the correct values.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.get_group_by_cols">
<code class="descname">get_group_by_cols</code>()<a class="headerlink" href="#django.db.models.Expression.get_group_by_cols" title="Permalink to this definition">¶</a></dt>
<dd><p>Responsible for returning the list of columns references by
this expression. <code class="docutils literal"><span class="pre">get_group_by_cols()</span></code> should be called on any
nested expressions. <code class="docutils literal"><span class="pre">F()</span></code> objects, in particular, hold a reference
to a column.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.asc">
<code class="descname">asc</code>()<a class="headerlink" href="#django.db.models.Expression.asc" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the expression ready to be sorted in ascending order.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.desc">
<code class="descname">desc</code>()<a class="headerlink" href="#django.db.models.Expression.desc" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the expression ready to be sorted in descending order.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.Expression.reverse_ordering">
<code class="descname">reverse_ordering</code>()<a class="headerlink" href="#django.db.models.Expression.reverse_ordering" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns <code class="docutils literal"><span class="pre">self</span></code> with any modifications required to reverse the sort
order within an <code class="docutils literal"><span class="pre">order_by</span></code> call. As an example, an expression
implementing <code class="docutils literal"><span class="pre">NULLS</span> <span class="pre">LAST</span></code> would change its value to be
<code class="docutils literal"><span class="pre">NULLS</span> <span class="pre">FIRST</span></code>. Modifications are only required for expressions that
implement sort order like <code class="docutils literal"><span class="pre">OrderBy</span></code>. This method is called when
<a class="reference internal" href="querysets.html#django.db.models.query.QuerySet.reverse" title="django.db.models.query.QuerySet.reverse"><code class="xref py py-meth docutils literal"><span class="pre">reverse()</span></code></a> is called on a
queryset.</p>
</dd></dl>
</dd></dl>
</div>
<div class="section" id="s-writing-your-own-query-expressions">
<span id="writing-your-own-query-expressions"></span><h3>Writing your own Query Expressions<a class="headerlink" href="#writing-your-own-query-expressions" title="Permalink to this headline">¶</a></h3>
<p>You can write your own query expression classes that use, and can integrate
with, other query expressions. Let’s step through an example by writing an
implementation of the <code class="docutils literal"><span class="pre">COALESCE</span></code> SQL function, without using the built-in
<a class="reference internal" href="#func-expressions"><span class="std std-ref">Func() expressions</span></a>.</p>
<p>The <code class="docutils literal"><span class="pre">COALESCE</span></code> SQL function is defined as taking a list of columns or
values. It will return the first column or value that isn’t <code class="docutils literal"><span class="pre">NULL</span></code>.</p>
<p>We’ll start by defining the template to be used for SQL generation and
an <code class="docutils literal"><span class="pre">__init__()</span></code> method to set some attributes:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">copy</span>
<span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">Expression</span>
<span class="k">class</span> <span class="nc">Coalesce</span><span class="p">(</span><span class="n">Expression</span><span class="p">):</span>
<span class="n">template</span> <span class="o">=</span> <span class="s1">'COALESCE( </span><span class="si">%(expressions)s</span><span class="s1"> )'</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">expressions</span><span class="p">,</span> <span class="n">output_field</span><span class="p">,</span> <span class="o">**</span><span class="n">extra</span><span class="p">):</span>
<span class="nb">super</span><span class="p">(</span><span class="n">Coalesce</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="n">output_field</span><span class="o">=</span><span class="n">output_field</span><span class="p">)</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">expressions</span><span class="p">)</span> <span class="o"><</span> <span class="mi">2</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'expressions must have at least 2 elements'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">expression</span> <span class="ow">in</span> <span class="n">expressions</span><span class="p">:</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">expression</span><span class="p">,</span> <span class="s1">'resolve_expression'</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">TypeError</span><span class="p">(</span><span class="s1">'</span><span class="si">%r</span><span class="s1"> is not an Expression'</span> <span class="o">%</span> <span class="n">expression</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">expressions</span> <span class="o">=</span> <span class="n">expressions</span>
<span class="bp">self</span><span class="o">.</span><span class="n">extra</span> <span class="o">=</span> <span class="n">extra</span>
</pre></div>
</div>
<p>We do some basic validation on the parameters, including requiring at least
2 columns or values, and ensuring they are expressions. We are requiring
<code class="docutils literal"><span class="pre">output_field</span></code> here so that Django knows what kind of model field to assign
the eventual result to.</p>
<p>Now we implement the pre-processing and validation. Since we do not have
any of our own validation at this point, we just delegate to the nested
expressions:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">resolve_expression</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">query</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">allow_joins</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">reuse</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">summarize</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">for_save</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="n">c</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span>
<span class="n">c</span><span class="o">.</span><span class="n">is_summary</span> <span class="o">=</span> <span class="n">summarize</span>
<span class="k">for</span> <span class="n">pos</span><span class="p">,</span> <span class="n">expression</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">expressions</span><span class="p">):</span>
<span class="n">c</span><span class="o">.</span><span class="n">expressions</span><span class="p">[</span><span class="n">pos</span><span class="p">]</span> <span class="o">=</span> <span class="n">expression</span><span class="o">.</span><span class="n">resolve_expression</span><span class="p">(</span><span class="n">query</span><span class="p">,</span> <span class="n">allow_joins</span><span class="p">,</span> <span class="n">reuse</span><span class="p">,</span> <span class="n">summarize</span><span class="p">,</span> <span class="n">for_save</span><span class="p">)</span>
<span class="k">return</span> <span class="n">c</span>
</pre></div>
</div>
<p>Next, we write the method responsible for generating the SQL:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">as_sql</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">compiler</span><span class="p">,</span> <span class="n">connection</span><span class="p">):</span>
<span class="n">sql_expressions</span><span class="p">,</span> <span class="n">sql_params</span> <span class="o">=</span> <span class="p">[],</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">expression</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">expressions</span><span class="p">:</span>
<span class="n">sql</span><span class="p">,</span> <span class="n">params</span> <span class="o">=</span> <span class="n">compiler</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">expression</span><span class="p">)</span>
<span class="n">sql_expressions</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">sql</span><span class="p">)</span>
<span class="n">sql_params</span><span class="o">.</span><span class="n">extend</span><span class="p">(</span><span class="n">params</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">extra</span><span class="p">[</span><span class="s1">'expressions'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">','</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">sql_expressions</span><span class="p">)</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">template</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">extra</span><span class="p">,</span> <span class="n">sql_params</span>
<span class="k">def</span> <span class="nf">as_oracle</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">compiler</span><span class="p">,</span> <span class="n">connection</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> Example of vendor specific handling (Oracle in this case).</span>
<span class="sd"> Let's make the function name lowercase.</span>
<span class="sd"> """</span>
<span class="bp">self</span><span class="o">.</span><span class="n">template</span> <span class="o">=</span> <span class="s1">'coalesce( </span><span class="si">%(expressions)s</span><span class="s1"> )'</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">as_sql</span><span class="p">(</span><span class="n">compiler</span><span class="p">,</span> <span class="n">connection</span><span class="p">)</span>
</pre></div>
</div>
<p>We generate the SQL for each of the <code class="docutils literal"><span class="pre">expressions</span></code> by using the
<code class="docutils literal"><span class="pre">compiler.compile()</span></code> method, and join the result together with commas.
Then the template is filled out with our data and the SQL and parameters
are returned.</p>
<p>We’ve also defined a custom implementation that is specific to the Oracle
backend. The <code class="docutils literal"><span class="pre">as_oracle()</span></code> function will be called instead of <code class="docutils literal"><span class="pre">as_sql()</span></code>
if the Oracle backend is in use.</p>
<p>Finally, we implement the rest of the methods that allow our query expression
to play nice with other query expressions:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_source_expressions</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">expressions</span>
<span class="k">def</span> <span class="nf">set_source_expressions</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">expressions</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">expressions</span> <span class="o">=</span> <span class="n">expressions</span>
</pre></div>
</div>
<p>Let’s see how it works:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">F</span><span class="p">,</span> <span class="n">Value</span><span class="p">,</span> <span class="n">CharField</span>
<span class="gp">>>> </span><span class="n">qs</span> <span class="o">=</span> <span class="n">Company</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span>
<span class="gp">... </span> <span class="n">tagline</span><span class="o">=</span><span class="n">Coalesce</span><span class="p">([</span>
<span class="gp">... </span> <span class="n">F</span><span class="p">(</span><span class="s1">'motto'</span><span class="p">),</span>
<span class="gp">... </span> <span class="n">F</span><span class="p">(</span><span class="s1">'ticker_name'</span><span class="p">),</span>
<span class="gp">... </span> <span class="n">F</span><span class="p">(</span><span class="s1">'description'</span><span class="p">),</span>
<span class="gp">... </span> <span class="n">Value</span><span class="p">(</span><span class="s1">'No Tagline'</span><span class="p">)</span>
<span class="gp">... </span> <span class="p">],</span> <span class="n">output_field</span><span class="o">=</span><span class="n">CharField</span><span class="p">()))</span>
<span class="gp">>>> </span><span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">qs</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">%s</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="n">c</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">c</span><span class="o">.</span><span class="n">tagline</span><span class="p">))</span>
<span class="gp">...</span>
<span class="go">Google: Do No Evil</span>
<span class="go">Apple: AAPL</span>
<span class="go">Yahoo: Internet Company</span>
<span class="go">Django Software Foundation: No Tagline</span>
</pre></div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="yui-b" id="sidebar">
<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
<div class="sphinxsidebarwrapper">
<h3><a href="../../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Query Expressions</a><ul>
<li><a class="reference internal" href="#supported-arithmetic">Supported arithmetic</a></li>
<li><a class="reference internal" href="#some-examples">Some examples</a></li>
<li><a class="reference internal" href="#built-in-expressions">Built-in Expressions</a><ul>
<li><a class="reference internal" href="#f-expressions"><code class="docutils literal"><span class="pre">F()</span></code> expressions</a><ul>
<li><a class="reference internal" href="#avoiding-race-conditions-using-f">Avoiding race conditions using <code class="docutils literal"><span class="pre">F()</span></code></a></li>
<li><a class="reference internal" href="#using-f-in-filters">Using <code class="docutils literal"><span class="pre">F()</span></code> in filters</a></li>
<li><a class="reference internal" href="#using-f-with-annotations">Using <code class="docutils literal"><span class="pre">F()</span></code> with annotations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#func-expressions"><code class="docutils literal"><span class="pre">Func()</span></code> expressions</a></li>
<li><a class="reference internal" href="#aggregate-expressions"><code class="docutils literal"><span class="pre">Aggregate()</span></code> expressions</a></li>
<li><a class="reference internal" href="#creating-your-own-aggregate-functions">Creating your own Aggregate Functions</a></li>
<li><a class="reference internal" href="#value-expressions"><code class="docutils literal"><span class="pre">Value()</span></code> expressions</a></li>
<li><a class="reference internal" href="#expressionwrapper-expressions"><code class="docutils literal"><span class="pre">ExpressionWrapper()</span></code> expressions</a></li>
<li><a class="reference internal" href="#conditional-expressions">Conditional expressions</a></li>
<li><a class="reference internal" href="#raw-sql-expressions">Raw SQL expressions</a></li>
</ul>
</li>
<li><a class="reference internal" href="#technical-information">Technical Information</a><ul>
<li><a class="reference internal" href="#expression-api">Expression API</a></li>
<li><a class="reference internal" href="#writing-your-own-query-expressions">Writing your own Query Expressions</a></li>
</ul>
</li>
</ul>
</li>
</ul>
<h3>Browse</h3>
<ul>
<li>Prev: <a href="lookups.html">Lookup API reference</a></li>
<li>Next: <a href="conditional-expressions.html">Conditional Expressions</a></li>
</ul>
<h3>You are here:</h3>
<ul>
<li>
<a href="../../index.html">Django 1.8.19 documentation</a>
<ul><li><a href="../index.html">API Reference</a>
<ul><li><a href="index.html">Models</a>
<ul><li>Query Expressions</li></ul>
</li></ul></li></ul>
</li>
</ul>
<div role="note" aria-label="source link">
<h3>This Page</h3>
<ul class="this-page-menu">
<li><a href="../../_sources/ref/models/expressions.txt"
rel="nofollow">Show Source</a></li>
</ul>
</div>
<div id="searchbox" style="display: none" role="search">
<h3>Quick search</h3>
<form class="search" action="../../search.html" method="get">
<div><input type="text" name="q" /></div>
<div><input type="submit" value="Go" /></div>
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
</div>
<script type="text/javascript">$('#searchbox').show(0);</script>
</div>
</div>
<h3>Last update:</h3>
<p class="topless">Mar 10, 2018</p>
</div>
</div>
<div id="ft">
<div class="nav">
« <a href="lookups.html" title="Lookup API reference">previous</a>
|
<a href="../index.html" title="API Reference" accesskey="U">up</a>
|
<a href="conditional-expressions.html" title="Conditional Expressions">next</a> »</div>
</div>
</div>
<div class="clearer"></div>
</div>
</body>
</html>
distrib
>
Mageia
>
6
>
armv7hl
>
media
>
core-updates
>
by-pkgid
>
65530c6176058f9b54858c3b4f6385e6
>
files
>
774
