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<h1>Identity Vs Equals</h1>
<p>One of the most common questions of db4o users is: why does
not db4o allow to use equals() and hashcode to identify objects in the
database. From the first glance it seems like a very attractive contract - let
the developer decide what should be the base for comparing objects and making
them unique in the database. For example if the database identity is based on
the object's field values it will prevent duplicate objects from being stored
to the database, as they will automatically be considered one object.</p>
<p>Yes, it looks attractive, but there is a huge pitfall: when
we deal with objects, we deal with their references to each other comprising a
unique object graph, which can be very complex. Preserving these references
becomes a task of storing many-to-many relationships. This task can only be
solved by providing unique identification to each object <b>in memory</b> and not only in the database, which means that it can't
depend on the information stored in the object (like an aggregate of field values).</p>
<p>To see it clearly, let's look at an example. Suppose we have
Pilot{string name} and Car{Pilot pilot} classes, and their <code>equals</code>
method is based on comparing field values:</p>
<ol><li>Store
a pilot1(name="name1") and car1(pilot=pilot1) to the database</li><li>Retrieve
pilot1</li><li>change
pilot1(name = "name1") to pilot1(name="name2"). Note that though it is the
same object from the runtime point of view, these are 2 different objects for
the database based on equals comparison.</li><li>Now
let's try to retrieve the car object, which has pilot = pilot1. We will
get no results as the initial pilot stored with the database is not equal
to the pilot1(name="name2"), and there is no car for the updated pilot
anymore!</li></ol><p>Now, this was a simple example, and can be solved by
updating the car object together with the pilot. But what happens if there are
thousands of objects referencing this pilot instance? They will all have to be
retrieved and updated. Further, those objects can be also referenced somewhere
and potentially a single update in a pilot object can trigger the re-write of
the whole database. </p>
<p>Objects without identity also make Transparent Persistence
and Activation impossible, as there will be no way to decide which instance is
the right one for update or activation.</p>
<p>So unique identification of database objects in memory is
unavoidable and identity based on an object reference is the most
straightforward way to get this identification.</p>
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This revision (2) was last Modified 2008-03-02T17:47:16 by Tetyana.
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