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    <p>I think you've actually found a bug in Entity Framework. I'm envious!</p> <p><strong>(EDIT: It looks like this bug has been fixed in EF 6.0.0-rc1)</strong></p> <p>Here's why I think it's a bug: the machinery which is translating the expression predicate is definitely misbehaving for <code>FirstOrDefault</code> in a way that does not occur with <code>Where</code>. I can repro your issue against a similar schema with these contrived queries:</p> <pre><code>// works var target = _context.Orders.Where (o =&gt; _context.Users.FirstOrDefault() != null).FirstOrDefault (); // Exception: Unable to create a constant value... target = _context.Orders.FirstOrDefault (o =&gt; _context.Users.FirstOrDefault() != null); </code></pre> <p>Not only does it throw an exception, but observing the SQL in LINQPad you can see that an effective <code>SELECT * FROM Users</code> query is being issued right before it blows up, as if it's trying to treat the <code>_context.Users.FirstOrDefault()</code> call as the <code>IEnumerable</code> version rather than the <code>IQueryable</code> version. Using an unmapped method as a predicate in the inner <code>FirstOrDefault</code> call results in the same "constant value" exception, rather than the "LINQ to Entities does not recognize the method" exception one would expect:</p> <pre><code>// throws "Unable to create a constant value..." exception! target = _context.Orders.FirstOrDefault (o =&gt; _context.Users.FirstOrDefault(u =&gt; MyBogusMethod(u)) != null); </code></pre> <p>It's as if the translator for <code>FirstOrDefault</code> is aggressively trying to reduce an expression of type <code>User</code> down to an instance of type <code>User</code> by executing the expression somehow, but before the reduction is complete (before the inner <code>FirstOrDefault</code> is evaluated) it realizes that the resulting type is not going to be allowed and blows up. Very strange.</p> <p>I briefly tried to figure out what's going wrong in the source, but it's way beyond me. <del>I'd recommend filing a bug with the EF guys: <a href="http://entityframework.codeplex.com/workitem/list/basic" rel="nofollow">http://entityframework.codeplex.com/workitem/list/basic</a></del></p> <p>In the meantime, it seems to work correctly as long as you put the predicate in the <code>Where</code> clause rather than the <code>FirstOrDefault</code>, even if you tag <code>FirstOrDefault</code> onto the end of your query:</p> <pre><code>var target = _context.Orders.Where(o =&gt; o.OrderID == order.OrderID &amp;&amp; (o.User.UserName.ToLower() == userName || _context.Users.FirstOrDefault(u =&gt; u.UserName == userName).Role.RoleName == "Administrator") ) .FirstOrDefault(); </code></pre> <p>(You might also consider storing the fact that the user has the "Administrator" role on the <code>User</code>, to avoid the need for complex queries like this.)</p>
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    1. PO"Unable to create a constant value of type 'User'" when using FirstOrDefault?
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    1. USSteve Ruble
    1. USSteve Ruble
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    1. PO"Unable to create a constant value of type 'User'" when using FirstOrDefault?
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    1. COAs an aside : Please try System.Web.HttpContext.Current.Request.LogonUserIdentity.Name instead of User.Identity.Name.
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      1. USGreg
    2. CO`.Where().FirstOrDefault()` did the trick. It encapsulates all the logic in a single query. Thanks!
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