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<div class="yui-g" id="topics-db-examples-many_to_one">
<div class="section" id="s-many-to-one-relationships">
<span id="many-to-one-relationships"></span><h1>Many-to-one relationships<a class="headerlink" href="#many-to-one-relationships" title="Permalink to this headline">ΒΆ</a></h1>
<p>To define a many-to-one relationship, use <a class="reference internal" href="../../../ref/models/fields.html#django.db.models.ForeignKey" title="django.db.models.ForeignKey"><tt class="xref py py-class docutils literal"><span class="pre">ForeignKey</span></tt></a>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.db</span> <span class="kn">import</span> <span class="n">models</span>
<span class="k">class</span> <span class="nc">Reporter</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">first_name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">30</span><span class="p">)</span>
<span class="n">last_name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">30</span><span class="p">)</span>
<span class="n">email</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">EmailField</span><span class="p">()</span>
<span class="c"># On Python 3: def __str__(self):</span>
<span class="k">def</span> <span class="nf">__unicode__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">u"</span><span class="si">%s</span><span class="s"> </span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">first_name</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">last_name</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Article</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">headline</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">100</span><span class="p">)</span>
<span class="n">pub_date</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">DateField</span><span class="p">()</span>
<span class="n">reporter</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">ForeignKey</span><span class="p">(</span><span class="n">Reporter</span><span class="p">)</span>
<span class="c"># On Python 3: def __str__(self):</span>
<span class="k">def</span> <span class="nf">__unicode__</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">headline</span>
<span class="k">class</span> <span class="nc">Meta</span><span class="p">:</span>
<span class="n">ordering</span> <span class="o">=</span> <span class="p">(</span><span class="s">'headline'</span><span class="p">,)</span>
</pre></div>
</div>
<p>What follows are examples of operations that can be performed using the Python
API facilities.</p>
<p>Create a few Reporters:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">r</span> <span class="o">=</span> <span class="n">Reporter</span><span class="p">(</span><span class="n">first_name</span><span class="o">=</span><span class="s">'John'</span><span class="p">,</span> <span class="n">last_name</span><span class="o">=</span><span class="s">'Smith'</span><span class="p">,</span> <span class="n">email</span><span class="o">=</span><span class="s">'john@example.com'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">r2</span> <span class="o">=</span> <span class="n">Reporter</span><span class="p">(</span><span class="n">first_name</span><span class="o">=</span><span class="s">'Paul'</span><span class="p">,</span> <span class="n">last_name</span><span class="o">=</span><span class="s">'Jones'</span><span class="p">,</span> <span class="n">email</span><span class="o">=</span><span class="s">'paul@example.com'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">r2</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
</pre></div>
</div>
<p>Create an Article:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">datetime</span> <span class="kn">import</span> <span class="n">date</span>
<span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="n">Article</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">headline</span><span class="o">=</span><span class="s">"This is a test"</span><span class="p">,</span> <span class="n">pub_date</span><span class="o">=</span><span class="n">date</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">27</span><span class="p">),</span> <span class="n">reporter</span><span class="o">=</span><span class="n">r</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">reporter</span><span class="o">.</span><span class="n">id</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">reporter</span>
<span class="go"><Reporter: John Smith></span>
</pre></div>
</div>
<p>Article objects have access to their related Reporter objects:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">r</span> <span class="o">=</span> <span class="n">a</span><span class="o">.</span><span class="n">reporter</span>
</pre></div>
</div>
<p>On Python 2, these are strings of type <tt class="docutils literal"><span class="pre">str</span></tt> instead of unicode strings
because that’s what was used in the creation of this reporter (and we haven’t
refreshed the data from the database, which always returns unicode strings):</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">first_name</span><span class="p">,</span> <span class="n">r</span><span class="o">.</span><span class="n">last_name</span>
<span class="go">('John', 'Smith')</span>
</pre></div>
</div>
<p>Create an Article via the Reporter object:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">new_article</span> <span class="o">=</span> <span class="n">r</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">create</span><span class="p">(</span><span class="n">headline</span><span class="o">=</span><span class="s">"John's second story"</span><span class="p">,</span> <span class="n">pub_date</span><span class="o">=</span><span class="n">date</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">29</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">new_article</span>
<span class="go"><Article: John's second story></span>
<span class="gp">>>> </span><span class="n">new_article</span><span class="o">.</span><span class="n">reporter</span>
<span class="go"><Reporter: John Smith></span>
<span class="gp">>>> </span><span class="n">new_article</span><span class="o">.</span><span class="n">reporter</span><span class="o">.</span><span class="n">id</span>
<span class="go">1</span>
</pre></div>
</div>
<p>Create a new article, and add it to the article set:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">new_article2</span> <span class="o">=</span> <span class="n">Article</span><span class="p">(</span><span class="n">headline</span><span class="o">=</span><span class="s">"Paul's story"</span><span class="p">,</span> <span class="n">pub_date</span><span class="o">=</span><span class="n">date</span><span class="p">(</span><span class="mi">2006</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">17</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">new_article2</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">new_article2</span><span class="o">.</span><span class="n">reporter</span>
<span class="go"><Reporter: John Smith></span>
<span class="gp">>>> </span><span class="n">new_article2</span><span class="o">.</span><span class="n">reporter</span><span class="o">.</span><span class="n">id</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">all</span><span class="p">()</span>
<span class="go">[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]</span>
</pre></div>
</div>
<p>Add the same article to a different article set - check that it moves:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">r2</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">new_article2</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">new_article2</span><span class="o">.</span><span class="n">reporter</span><span class="o">.</span><span class="n">id</span>
<span class="go">2</span>
<span class="gp">>>> </span><span class="n">new_article2</span><span class="o">.</span><span class="n">reporter</span>
<span class="go"><Reporter: Paul Jones></span>
</pre></div>
</div>
<p>Adding an object of the wrong type raises TypeError:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">r2</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="gr">TypeError</span>: <span class="n">'Article' instance expected</span>
<span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">all</span><span class="p">()</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
<span class="gp">>>> </span><span class="n">r2</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">all</span><span class="p">()</span>
<span class="go">[<Article: Paul's story>]</span>
<span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">count</span><span class="p">()</span>
<span class="go">2</span>
<span class="gp">>>> </span><span class="n">r2</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">count</span><span class="p">()</span>
<span class="go">1</span>
</pre></div>
</div>
<p>Note that in the last example the article has moved from John to Paul.</p>
<p>Related managers support field lookups as well.
The API automatically follows relationships as far as you need.
Use double underscores to separate relationships.
This works as many levels deep as you want. There’s no limit. For example:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">r</span><span class="o">.</span><span class="n">article_set</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">headline__startswith</span><span class="o">=</span><span class="s">'This'</span><span class="p">)</span>
<span class="go">[<Article: This is a test>]</span>
<span class="go"># Find all Articles for any Reporter whose first name is "John".</span>
<span class="gp">>>> </span><span class="n">Article</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">reporter__first_name__exact</span><span class="o">=</span><span class="s">'John'</span><span class="p">)</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
</pre></div>
</div>
<p>Exact match is implied here:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Article</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">reporter__first_name</span><span class="o">=</span><span class="s">'John'</span><span class="p">)</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
</pre></div>
</div>
<p>Query twice over the related field. This translates to an AND condition in the
WHERE clause:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Article</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">reporter__first_name__exact</span><span class="o">=</span><span class="s">'John'</span><span class="p">,</span> <span class="n">reporter__last_name__exact</span><span class="o">=</span><span class="s">'Smith'</span><span class="p">)</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
</pre></div>
</div>
<p>For the related lookup you can supply a primary key value or pass the related
object explicitly:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Article</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">reporter__pk</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
<span class="gp">>>> </span><span class="n">Article</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">reporter</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
<span class="gp">>>> </span><span class="n">Article</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">reporter</span><span class="o">=</span><span class="n">r</span><span class="p">)</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
<span class="gp">>>> </span><span class="n">Article</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">reporter__in</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">])</span><span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="go">[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]</span>
<span class="gp">>>> </span><span class="n">Article</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">reporter__in</span><span class="o">=</span><span class="p">[</span><span class="n">r</span><span class="p">,</span><span class="n">r2</span><span class="p">])</span><span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="go">[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]</span>
</pre></div>
</div>
<p>You can also use a queryset instead of a literal list of instances:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Article</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">reporter__in</span><span class="o">=</span><span class="n">Reporter</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">first_name</span><span class="o">=</span><span class="s">'John'</span><span class="p">))</span><span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="go">[<Article: John's second story>, <Article: This is a test>]</span>
</pre></div>
</div>
<p>Querying in the opposite direction:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Reporter</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">article__pk</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="go">[<Reporter: John Smith>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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">article</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="go">[<Reporter: John Smith>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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">article</span><span class="o">=</span><span class="n">a</span><span class="p">)</span>
<span class="go">[<Reporter: John Smith>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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">article__headline__startswith</span><span class="o">=</span><span class="s">'This'</span><span class="p">)</span>
<span class="go">[<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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">article__headline__startswith</span><span class="o">=</span><span class="s">'This'</span><span class="p">)</span><span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="go">[<Reporter: John Smith>]</span>
</pre></div>
</div>
<p>Counting in the opposite direction works in conjunction with distinct():</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Reporter</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">article__headline__startswith</span><span class="o">=</span><span class="s">'This'</span><span class="p">)</span><span class="o">.</span><span class="n">count</span><span class="p">()</span>
<span class="go">3</span>
<span class="gp">>>> </span><span class="n">Reporter</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">article__headline__startswith</span><span class="o">=</span><span class="s">'This'</span><span class="p">)</span><span class="o">.</span><span class="n">distinct</span><span class="p">()</span><span class="o">.</span><span class="n">count</span><span class="p">()</span>
<span class="go">1</span>
</pre></div>
</div>
<p>Queries can go round in circles:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Reporter</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">article__reporter__first_name__startswith</span><span class="o">=</span><span class="s">'John'</span><span class="p">)</span>
<span class="go">[<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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">article__reporter__first_name__startswith</span><span class="o">=</span><span class="s">'John'</span><span class="p">)</span><span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="go">[<Reporter: John Smith>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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">article__reporter__exact</span><span class="o">=</span><span class="n">r</span><span class="p">)</span><span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="go">[<Reporter: John Smith>]</span>
</pre></div>
</div>
<p>If you delete a reporter, his articles will be deleted (assuming that the
ForeignKey was defined with <a class="reference internal" href="../../../ref/models/fields.html#django.db.models.ForeignKey.on_delete" title="django.db.models.ForeignKey.on_delete"><tt class="xref py py-attr docutils literal"><span class="pre">django.db.models.ForeignKey.on_delete</span></tt></a> set to
<tt class="docutils literal"><span class="pre">CASCADE</span></tt>, which is the default):</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Article</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="go">[<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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="s">'first_name'</span><span class="p">)</span>
<span class="go">[<Reporter: John Smith>, <Reporter: Paul Jones>]</span>
<span class="gp">>>> </span><span class="n">r2</span><span class="o">.</span><span class="n">delete</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">Article</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="go">[<Article: John's second story>, <Article: This is a test>]</span>
<span class="gp">>>> </span><span class="n">Reporter</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="s">'first_name'</span><span class="p">)</span>
<span class="go">[<Reporter: John Smith>]</span>
</pre></div>
</div>
<p>You can delete using a JOIN in the query:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">Reporter</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">article__headline__startswith</span><span class="o">=</span><span class="s">'This'</span><span class="p">)</span><span class="o">.</span><span class="n">delete</span><span class="p">()</span>
<span class="gp">>>> </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="go">[]</span>
<span class="gp">>>> </span><span class="n">Article</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="go">[]</span>
</pre></div>
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