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POLINQ-to-entities generic == workaround
 

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    <p>I have a following LINQ-to-entities query</p> <pre><code>IQueryable&lt;History&lt;T&gt;&gt; GetFirstOperationsForEveryId&lt;T&gt; (IQueryable&lt;History&lt;T&gt;&gt; ItemHistory) { var q = (from h in ItemHistory where h.OperationId == (from h1 in ItemHistory where h1.GenericId == h.GenericId select h1.OperationId).Min() select h); return q; } </code></pre> <p><code>ItemHistory</code> is a generic query. It can be obtained in the following way</p> <pre><code>var history1 = MyEntitiySet1.Select(obj =&gt; new History&lt;long&gt;{ obj.OperationId, GenericId = obj.LongId }); var history2 = AnotherEntitiySet.Select(obj =&gt; new History&lt;string&gt;{ obj.OperationId, GenericId = obj.StringId }); </code></pre> <p>In the end of all I want a generic query being able to work with any entity collection convertible to <code>History&lt;T&gt;</code>.</p> <p>The problem is the code does <strong>not compile</strong> because of GenericId comparison in the inner query (Operator '==' cannot be applied to operands of type 'T' and 'T').</p> <p>If I change == to <code>h1.GenericId.Equals(h.GenericId)</code> I get the following <code>NotSupportedException</code>:</p> <blockquote> <p>Unable to cast the type 'System.Int64' to type 'System.Object'. LINQ to Entities only supports casting Entity Data Model primitive types.</p> </blockquote> <p>I've tried to do grouping instead of subquery and join the results.</p> <pre><code>IQueryable&lt;History&lt;T&gt;&gt; GetFirstOperationsForEveryId&lt;T&gt; (IQueryable&lt;History&lt;T&gt;&gt; ItemHistory) { var grouped = (from h1 in ItemHistory group h1 by h1.GenericId into tt select new { GenericId = tt.Key, OperationId = tt.Min(ttt =&gt; ttt.OperationId) }); var q = (from h in ItemHistory join g in grouped on new { h.OperationId, h.GenericId } equals new { g.OperationId, g.GenericId } select h); return q; } </code></pre> <p>It compiles because GenericId's are compared with <code>equals</code> keyword and it works but the query with real data is too slow (it has been running for 11 hours on dedicated postgresql server).</p> <p>There is an option to build a whole expression for the outer where statement. But the code would be too long and unclear.</p> <p>Are there any simple workarounds for equality comparison with generics in LINQ-to-entities?</p>
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