Extending LINQ to Nhibernate provider, in combination with Dynamic LINQ problem

Question

I'm using NHibernate 3.1.0 and I'm trying to extend the LINQ provider by using BaseHqlGeneratorForMethod and extending the DefaultLinqToHqlGeneratorsRegistry as explained in Fabio's post.

For example, to support ToString() I've created a ToStringGenerator as below.

internal class ToStringGenerator : BaseHqlGeneratorForMethod
{
    public ToStringGenerator()
    {
        SupportedMethods = new[]
            {
                ReflectionHelper.GetMethodDefinition<object>(x => x.ToString())
            };
    }

    public override HqlTreeNode BuildHql(MethodInfo method, Expression targetObject, ReadOnlyCollection<Expression> arguments, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
    {
        return treeBuilder.Cast(visitor.Visit(targetObject).AsExpression(), typeof(string));
    }
}

and I have registered using

internal class CustomLinqToHqlGeneratorsRegistry : DefaultLinqToHqlGeneratorsRegistry
{
    public CustomLinqToHqlGeneratorsRegistry()
    {
        this.Merge(new ToStringGenerator());
    }
}

etc. So far this works for "static" queries, I can use it like this:

var results = mSession.Query<Project>();
string pId = "1";
results = results.Where(p => p.Id.ToString().Contains(pId));

This translates correctly to its SQL counterpart (using SQL Server 2008)

where cast(project0_.Id as NVARCHAR(255)) like (''%''+@p0+''%'')

The problem arises when I try to use it in combination with Microsoft Dynamic LINQ library (discussed in this Scott Guthrie's post) like this:

var results = mSession.Query<Project>();
string pId = "1";
results = results.Where("Id.ToString().Contains(@0)", pId);

This results in a NotSupportedException with a message of "System.String ToString()" (which was the exact same messages I was getting with the static queries before implementing the classes mentioned above). This exception is being thrown with a source of "NHibernate" and with the StackTrace at "at NHibernate.Linq.Visitors.HqlGeneratorExpressionTreeVisitor.VisitMethodCallExpression(MethodCallExpression expression)".

So what am I missing here? What have I done wrong, or what needs to be done to support this scenario?

Answer 1

I had the same problem and fixed it.
At first I want to thank murki for providing the information which got me on my way!

The answer lies partly in Fabio's post. To solve this issue you have to use the RegisterGenerator instead of the Merge method in the CustomLinqToHqlGeneratorsRegistry constructor. My implementation of the CustomLinqToHqlGeneratorsRegistry class is as follows:

public class CustomLinqToHqlGeneratorsRegistry : DefaultLinqToHqlGeneratorsRegistry
{
    public CustomLinqToHqlGeneratorsRegistry()
        : base()
    {
        MethodInfo toStringMethod = ReflectionHelper.GetMethodDefinition<int>(x => x.ToString());
        RegisterGenerator(toStringMethod, new ToStringGenerator());
    }
}

Answer 2

There are two, well defined, separate stages here:

1. Converting the dynamic (string) query into a static expression (done by the Dynamic Linq library)
2. Parsing that into an HqlTree, then executing (done by NHibernate)

Since you have determined that a static expression works well, the problem lies in 1.

What happens if you do the following?

var results = Enumerable.Empty<Project>().AsQueryable();
string pId = "1";
results = results.Where("Id.ToString().Contains(@0)", pId);

If it fails, you'll have confirmed it's a problem with Dynamic Linq alone (i.e. it doesn't support the expression you're feeding it), so you'll have to dig into it and patch it.

Semi-related: the ToStringGenerator looks useful; could you submit a patch for NHibernate? http://jira.nhforge.org