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<h1>Identity And Primary Keys</h1>
<p>One of the most important concepts to understand working
with a database system is the identification strategy. </p>
<p>In RDBMS separate entities are distinguished with the help
of primary keys. Effectively, objects are compared on their field contents and
a constrain is used prohibiting 2 different rows in a table to have the same
value in a column marked as a primary key. </p>
<p>In the object world, the concept is quite different - any
object is unique, independently of the data it holds. For example:</p>
<p><code>class Car </code></p>
<p><code>
model</code></p>
<p><code> year</code></p>
<p><code>car1 = new Car("BMW","1999") </code></p>
<p><code>car2 = new Car("BMW","1999") </code></p>
<p><code>car1</code> and <code>car2</code> objects consist of
the same data, but they are not equal.</p>
<p>On the other hand we can create any amount of references to
the object and they will all be equal:</p>
<p><code>car1 = new Car("BMW","1999") </code></p>
<p><code>car2 = car1</code></p>
<p><code>car3 = car1</code></p>
<p>In this example <code>car2</code> is equal to
<code>car1</code> and <code>car3</code> and all these variables actually
reference the same object.</p>
<p>Object database task is to ensure that independently of the
way the object was accessed (by different queries or through different navigation
access paths) the database must return a reference to the one single object.
For a detailed explanation how it is achieved in db4o, please see <a href="../../object_identity/unique_identity_concept.html" class="wikiLink">Unique identity concept</a>.</p>
<p>If you are moving from a relational database to db4o it is
very important to understand db4o identity concept. Instead of tying objects to
the database reference key you should use the objects, obtained from the
database directly without additional worries about their identity.</p>
<p>Unfortunately, there still are some cases, when additional
identification system is necessary. This can happen when an object must be
referenced out of the application memory boundaries: objects referenced between
sessions, objects passed to another application, partially connected applications
etc. In these cases, you are encouraged to use db4o internal IDs or Unique
Universal IDs (UUID). Internal IDs are unique within one database file and do
change after defragment. UUIDs are unique between all db4o databases and do not
change. For more information see <a href="../../../implementation_strategies/ids_and_uuids.html" class="wikiLink">IDs and UUIDs</a>.</p>
If your application requires autoincremented ID
generation you can look at an example implementation in <a href="../../../implementation_strategies/callbacks/possible_usecases/autoincrement.html" class="wikiLink">Autoincrement</a>
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This revision (1) was last Modified 2007-06-02T20:21:13 by Tetyana.
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