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POReconciling DDD, viewmodels, and performance
 

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    <p>I am starting to learn about DDD and am concerned about the performance implications of retrieving entity objects from persistence and then restructuring them in a viewmodel for the UI.</p> <p>Let's say that I have two aggregate roots:</p> <pre><code>Person Orders ------ ------- personId orderId name personId </code></pre> <p>Each aggregate root has its own repository in charge of basic CRUD operations for the entire aggregate. </p> <p>Let's say that the UI requires the following columns:</p> <pre><code>viewmodel --------- personName numberOfOrders </code></pre> <p>I can think of two ways that I can populate this viewmodel:</p> <ol> <li>Eagerly load all person entities, eagerly load all orders based on personId, restructure loaded entities into the viewmodel.</li> <li>create a JOIN/COUNT(orderId) stored procedure and have the database return data in the same structure as the viewmodel.</li> </ol> <p>Obviously, option 1 can be quite the expensive operation, as there can be multiple persons and multiple orders resulting in MULTIPLE database calls. Option 2 will require only one database call.</p> <p>If Option 2 is the preferred (performant) option, where do I store this "viewmodel" and the so called "database call?" Is there a separate "data service layer" on top of the repositories that I can implement? Or is that an anti-pattern in regards to how DDD is generally implemented?</p> <p>Basically, how do I reconcile complex DDD aggregates with custom UI Viewmodels keeping performance in mind?</p> <h2>UPDATES</h2> <p><strong>Specifications / Query Object</strong></p> <p>In talks with a friend, he had suggested that a possible solution is some sort of specification/query object pattern. The only problem is that we will have to implement this on the repository-level, requiring me to combine the Persons and Orders into one large aggregate. This is something I generally avoid for transaction consistency reasons.</p>
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