NHibernate ThenFetchMany is retrieving duplicate children

Question

I have a parent object with a child collection containing one element, the child collection contains a "grandchild" collection containing 3 elements.

I am loading the parent object from the database using NHibernate as follows

Parent parentObject = session.Query<Parent>()
    .FetchMany(x => x.Children)
    .ThenFetchMany(x => x.GrandChildren)
    .Where(x => x.Id = "someparentid")
    .Single();

What I'm finding is that there are duplicate children objects (3 in total) attached to the parent object when there should be only one. (There are 3 grandchild objects correctly attached to each child.) Eager loading the children collection only works correctly.

Do you know how I can achieve loading of the full parent object without duplicate children?

Answer 1

If you map Children and GrandChildren as set, you can avoid the cartesian product. You need to define Children and Grandchildren as collections:

public class Parent
{
    ...
    public virtual ICollection<Child> Children { get; set; }
    ...
}

public class Child
{
    ...
    public virtual ICollection<GrandChild> GrandChildren { get; set; }
    ...
}

And in the mapping (using FluentNHibernate):

public class ParentMapping : ClassMap<Parent>
{
    public ParentMapping()
    {
        ...
        HasMany(x => x.Children)
            .KeyColumn("ParentID")
            .Inverse
            .AsSet()
        ...
    }
}

public class ChildMapping : ClassMap<Child>
{
    public ChildMapping()
    {
        ...
        HasMany(x => x.GrandChildren)
            .KeyColumn("ChildID")
            .Inverse
            .AsSet()
        ...
    }
}

Answer 2

I was able to use the answer here using QueryOver, it correctly loads the objects while generating efficient SQL (selects per table instead of one huge join).

Answer 3

If you are using Linq, you can simplify it with this:

int parentId = 1;
var p1 = session.Query<Parent>().Where(x => x.ParentId == parentId);

p1
.FetchMany(x => x.Children)
.ToFuture();

sess.Query<Child>()
.Where(x => x.Parent.ParentId == parentId);
.FetchMany(x => x.GrandChildren)
.ToFuture();

Parent p = p1.ToFuture().Single();

Detailed explanation here: http://www.ienablemuch.com/2012/08/solving-nhibernate-thenfetchmany.html