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POSet operations in predicates passed to NHibernate's Criteria API
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  1. POSet operations in predicates passed to NHibernate's Criteria API
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    copied!<p>In the project I am working on, we are passing a specification (ala the Specification pattern as put forward by Martin Fowler) to a repository that uses NHibernate as its backend. The predicates to determine whether a given object are implemented as lambdas wrapped as Expressions, which the repository hands off to the Criteria API in NHibernate to actually fetch the result.</p> <p>What we have looks something like:</p> <pre><code>class MySpecification : ISpecification&lt;AnEntity&gt; { public Expression&lt;Func&lt;AnEntity, bool&gt;&gt; Satisfies { get { return entity =&gt; entity.Children.Count(c =&gt; !c.IsDeleted) == 0; } } } </code></pre> <p>and in the repository:</p> <pre><code>public IEnumerable&lt;AnEntity&gt; Select(ISpecification&lt;AnEntity&gt; spec) { ICriteria criteria = Session.CreateCriteria&lt;AnEntity&gt;(); criteria.Add(NHibernate.Criterion.Expression.Where(spec.Satisfies)); return criteria.List&lt;AnEntity&gt;(); } </code></pre> <p>At runtime, we get exceptions of the form: </p> <pre><code>{variable 'entity' of type 'AnEntity' referenced from scope '', but it is not defined"} System.Exception {System.InvalidOperationException} </code></pre> <p>When we reduce the predicates to simple boolean expressions that perform no set operations (e.g. Any, Count, Where) on child collections, everything works fine. </p> <p>With that in mind, is there any way to build a criterion using a lambda expression that operates on the child collections of the main entity?</p>
 

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