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POUsing the Data Mapper Pattern, Should the Entities (Domain Objects) know about the Mapper?
 

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    <p>I'm working with Doctrine2 for the first time, but I think this question is generic enough to not be dependent on a specific ORM. </p> <p><strong>Should the entities in a Data Mapper pattern be aware - and <em>use</em> - the Mapper</strong>?</p> <p>I have a few specific examples, but they all seem to boil down to the same general question. </p> <p>If I'm dealing with data from an external source - for example a <code>User</code> has many <code>Messages</code> - and the external source simply provides the latest few entities (like an RSS feed), how can <code>$user-&gt;addMessage($message)</code> check for duplicates unless it either is aware of the Mapper, or it 'searches' through the collection (seems like an inefficient thing to do).</p> <p>Of course a Controller or Transaction Script could check for duplicates before adding the message to the user - but that doesn't seem quite right, and would lead to code duplication.</p> <p>If I have a large collection - again a <code>User</code> with many <code>Messages</code> - how can the <code>User</code> entity provide limiting and pagination for the collection without actually proxying a Mapper call?</p> <p>Again, the Controller or Transaction Script or whatever is using the Entity could use the Mapper directly to retrieve a collection of the <code>User</code>'s <code>Messages</code> limited by count, date range, or other factors - but that too would lead to code duplication. </p> <p><strong>Is the answer using Repositories and making the Entity aware of them</strong>? (At least for Doctrine2, and whatever analogous concept is used by other ORMs.) At that point the Entity is still relatively decoupled from the Mapper.</p>
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