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POC# and LINQ: ordering a query by a nested query's value
 

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    <p>I'm writing a simple forum in ASP.NET that sits on top of an Entity Framework database in C#.</p> <p>Each <code>Topic</code> object has a navigation property <code>Posts</code> pointing to a collection of <code>Post</code> objects. Each <code>Post</code> object has a property <code>When</code> that indicates when the post was made.</p> <p><code>Post.Parent</code> is type <code>Topic</code>. <code>Post.Identifier</code> and <code>Topic.Identifier</code> is type <code>Int32</code>. Only <code>Post</code> has a <code>When</code> property; <code>Topic</code> has no such property. <code>Topic.Parent</code> is a third type <code>Forum</code> that is referenced by its <code>Identifier</code>.</p> <p>My problem is this: I can't seem to find a way to sort all Topic objects by the <em>last</em> post made in each topic. I've tried this:</p> <pre class="lang-cs prettyprint-override"><code>var topics = from t in context.Topics from p in a.Posts where t.Parent.Identifier == forum.Identifier orderby p.When descending select t; </code></pre> <p>But I got duplicate <code>Topic</code> objects and sorting was <em>not</em> by latest post date descending. </p> <p>I then tried this:</p> <pre class="lang-cs prettyprint-override"><code>var topics = (from t in context.Topics let lastPost = (from p in context.Posts where p.Parent.Identifier == a.Identifier orderby p.When descending select p).FirstOrDefault().When where t.Parent.Identifier == forum.Identifier orderby lastPost descending select t).Distinct(); </code></pre> <p>It eliminated the duplicate problem, but still no sorting. I even tried <a href="https://stackoverflow.com/questions/931617/linq-query-for-a-forum">a suggestion from another question</a>:</p> <pre class="lang-cs prettyprint-override"><code>var topics = (from t in context.Topics let posts = context.Posts.Where(p =&gt; p.Parent.Identifier == t.Identifier) let lastPost = posts.OrderByDescending(p =&gt; p.When).FirstOrDefault() where t.Parent.Identifier == forum.Identifier orderby lastPost.When descending select t); </code></pre> <p>Not sure what to try next; it seems that these more advanced LINQ expressions escape me. Any ideas?</p>
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