NHibernate: HqlGenerator: Create calculated property by concatenating other properties

Question

I am trying to add an HqlGenerator to allow NHibernate to query a calculated property on the following domain entity:

public class User 
{
    public string FirstName { get; set; }
    public string LastName {get; set; }
    public string FullName
    {
        get { return FirstName + " " + LastName; }
    }
}

I have created an HQL generator like so:

public class UserFullName : BaseHqlGeneratorForProperty
{
    public UserFullName()
    {
        var properties = new List<MemberInfo> { typeof(User).GetProperty("FullName") };
        SupportedProperties = properties;
    }

    public override HqlTreeNode BuildHql(MemberInfo member, Expression expression, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
    {
        var user = visitor.Visit(expression).AsExpression();
        return treeBuilder.Concat(
            treeBuilder.Dot(user, treeBuilder.Ident("FirstName")),
            treeBuilder.Constant(" "),
            treeBuilder.Dot(user, treeBuilder.Ident("LastName"))
        );
    }
}

The generator is wired up correctly into the configuration as the debugger will break within the BuildHql method when I run the query:

session.Query<User>().FirstOrDefault(x => x.FullName == "Aaron Janes");

however, later on in the NHibernate internals an exception is thrown:

NHibernate.Hql.Ast.ANTLR.QuerySyntaxException : Exception of type 'Antlr.Runtime.NoViableAltException' was thrown. [.FirstOrDefault[User](NHibernate.Linq.NhQueryable`1[User], Quote((x, ) => (String.op_Equality(x.FullName, Aaron Janes))), )]

Can anyone spot what I am doing wrong?

Answer 1

In your BuildHQL() you include the subexpression 'user' multiple times. This is not possible. You need to use

var user1 = visitor.Visit(expression).AsExpression();
var user2 = visitor.Visit(expression).AsExpression();

and use each of those once in the Concat expression.