Using Linq expressions as a specification pattern with parent/child query

Question

I'm trying to use the specification pattern implemented as a Linq expression so that Linq providers can parse it to produce efficient database queries.

This gives the basic idea.

I am having a hard time trying trying to get it working with a parent/child query

class Parent
{
    public int Foo;

    public IList<Child> Children = new List<Child>();
}

class Child
{
    public int Bar;
}

class Program
{
    static void Main(string[] args)
    {
        IQueryable<Parent> qry = GetQry(); //initialised


        //This works but duplicates the IsBigBar() logic
        //Included to show what I am trying to query on
        var parentsWithBigChildBars =
                from parents in qry
                where parents.Children.Any(child => child.Bar > 10) 
                select parents;

        var parentsWithBigChildBars2 =
               from parents in qry
               where parents.Children.Any( ?? ) //but how do i access my IsBigBar() expression from here?
               select parents;
    }


    //I want to re-use it to pull parents back!
    public Expression<Func<Child, bool>> IsBigBar()
    {
        return child => child.Bar > 10;
    }

    //I'f i use this as the Any() delegate, it compiles & runs but not an expression so evaluated client side
    public Func<Child, bool> IsBigBar2()
    {
        return child => child.Bar > 10;
    }
}

Answer 1

You want:

    var predicate = IsBigBar();
    var parentsWithBigChildBars2 =
           from parents in qry
           where parents.Children.Any(predicate) 
           select parents;

The extra var is very important. It prevents the query provider (that owns qry) from trying to interpret IsBigBar() and instead points it at the result of that method.