NHibernate + IQueryable access subtype property in where

Question

I've got problem with NHibernate v3.2.0, and following query:

class DocumentBase {}
class Requisition: DocumentBase {}
class Order: DocumentBase {}

Repository.GetAll<DocumentBase>()
    .Where(d => (d is Requisition) && ((Requisition)d).ProductItem != null)

Base query is designed to list all documents, but there is also possibility to filter this documents by type (and quasi-type, which is for example document without product). In code above there is only one condition, but predicate can be more complicated, for ex:

Repository.GetAll<DocumentBase>()
    .Where(d => 
        ((d is Requisition) && ((Requisition)d).ProductItem != null) ||
        (d is Order) ||
        [...]
    )

When executed I receive InvalidPathException with message Invalid path: 'd.ProductItem'. Any ideas? Is it supported?

So far I managed to bypass this error with following query:

Repository.GetAll<DocumentBase>()
    .Where(d => 
        (d is Requisition) &&
        Repository.GetAll<Requisition>()
            .Any(r => r.Id == d.Id && r.ProductItem != null)
    )

But definitely it's not the best choise in terms of performance.

Downcasts with filters on the subtype are not supported. – Diego Mijelshon Aug 09 '12 at 13:32 — Diego Mijelshon, Aug 09 '12 at 13:32

score 2 · Answer 1 · answered Aug 08 '12 at 08:47

2

I guess that your Repository.GetAll<T> method is returning session.Query<T>. There is only a limited subset of expressions you can do have your Where clause. NHibernate tries to convert your Where expression to HQL, and eventually to SQL. This means that you cannot write anything in Where and expect it to work, but only those expressions for which the conversion exists. However, NHibernate's Linq provider can be extended.

Your case can be simplified. If you have your subclasses mapped... and you probably do, the query can be written as:

Repository.GetAll<Requisition>().Where(r => r.ProductItem != null);

answered Aug 08 '12 at 08:47

Miroslav Popovic

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I know that there are not supported expressions, but when used then UnsupportedException is raised. Using `Repository.GetAll()` is not possible, because I need to support also other types. – Krzysztof Aug 08 '12 at 08:54
@Lolo Could you elaborate a little more on support for other types? Maybe update your question? – Miroslav Popovic Aug 08 '12 at 09:15

score 0 · Accepted Answer · answered Sep 13 '12 at 21:18

As someone mentioned it is possible to extend Linq provider in NHibernate. Here is solution which worked for me:

public static class OzirNhExtensions
{
    // Cast method to use in query
    public static TTarget Cast<TTarget>(this Object source)
    {
        return ((TTarget)source);
    }
}

class CastHqlGeneratorForMethod : BaseHqlGeneratorForMethod
{
    public CastHqlGeneratorForMethod()
    {
        this.SupportedMethods = new MethodInfo[] {
            ReflectionHelper.GetMethodDefinition(
                () => OzirNhExtensions.Cast<Object>(null)
            )
        };
    }

    // In here simply skip cast expression 
    // (it works probably because I have every sub-entity 
    // in the same table that base entity)
    public override HqlTreeNode BuildHql(
        MethodInfo method, 
        Expression targetObject, 
        ReadOnlyCollection<Expression> arguments, 
        HqlTreeBuilder treeBuilder,
        IHqlExpressionVisitor visitor)
    {
        return visitor.Visit(arguments[0]).AsExpression();
    }
}

Example query:

Repository.GetAll<DocumentBase>()
    .Where(d => d.Cast<Requisition>().ProductItem != null && 
        d.Cast<Requisition>().ProductItem.Name == "Something"
    )

NHibernate + IQueryable access subtype property in where

2 Answers2

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