NHibernate / MySQL string concatenation

Question

I have a nhibernate linq query that looks like this:

 from b in session.Query<Bookmark>()
where b.Uri.Equals(uri) ||
      b.Uri.Equals("www." + uri) ||
string.Concat("www.", b.Uri).Equals(uri)
select b

This blows up, saying Concat isn't support, but when I change it to

 from b in session.Query<Bookmark>()
where b.Uri.Equals(uri) ||
      b.Uri.Equals("www." + uri) ||
      ("www." + b.Uri).Equals(uri)
select b

It runs fine, but the query looks like this:

select cast(count(*) as SIGNED) as col_0_0_ 
 from bookmarks bookmark0_ 
 where bookmark0_.Uri = 'www.google.com' 
    or bookmark0_.Uri = 'www.www.google.com'
    or 'www.'+bookmark0_.Uri = 'www.google.com';

Where the 'www.'+bookmark0_.Uri is "added" instead of concat('www.',bookmark0_.Uri). Is there a way to concatenate strings in Linq for NHibernate for MySQL?

Answer 1

The following is an HqlGenerator that solves this problem:

public class ConcatHqlGenerator : BaseHqlGeneratorForMethod
{
    public ConcatHqlGenerator()
        : base()
    {
        this.SupportedMethods = new[] 
        { ReflectionHelper.GetMethodDefinition(() => string.Concat(null, null)) };
    }

    public override HqlTreeNode BuildHql(MethodInfo method,
Expression targetObject,
ReadOnlyCollection<Expression> arguments,
HqlTreeBuilder treeBuilder,
IHqlExpressionVisitor visitor)
    {
        return treeBuilder.Concat(
            new[] 
            {
                visitor.Visit(arguments[0]).AsExpression(),
                visitor.Visit(arguments[1]).AsExpression()
            });
    }
}

Add this to your HQLGeneratorsRegistry and you will be good to go with calls to string.Concat in you LINQ statements.

public class LinqToHqlGeneratorsRegistry : DefaultLinqToHqlGeneratorsRegistry
{
    public LinqToHqlGeneratorsRegistry()
        : base()
    {
        this.Merge(new ConcatHqlGenerator());
    }
}
private static ISessionFactory CreateSessionFactory()
{
    var configuration = new NHib.Cfg.Configuration();
    configuration.Properties.Add(NHibernate.Cfg
                                           .Environment.LinqToHqlGeneratorsRegistry, 
typeof(LinqToHqlGeneratorsRegistry).AssemblyQualifiedName);
    configuration.Configure();
    return configuration.BuildSessionFactory();
}