Can NHibernate query for specific children without lazy loading the entire collection?

Question

When I have an entity object with a one-to-many child collection, and I need to query for a specific child object, is there a feature or some clever pattern I haven't come up with yet to avoid that NHibernate fetches the entire child collection?

Example:

class Parent 
{
    public virtual int Id { get; proteced set; } // generated PK
    public virtual IEnumerable<Child> Children { get; proteced set; }
}

class Child
{
    public virtual int Id { get; protected set; } // generated PK
    public virtual string Name { get; protected set; }
    public virtual Parent Parent { get; protected set; }
}

// mapped with Fluent

class Service
{

    private readonly ISessionFactory sessionFactory;

    public Service(ISessionFactory sessionFactory)
    {
        this.sessionFactory = sessionFactory;
    }

    void DoSomethingWithChildrenNamedBob(int parentId)
    {
        using(var session = sessionFactory.OpenSession())
        {
            var parent = session.Get<Parent>(parentId);
            // Will cause lazy fetch of all children!
            var childrenNamedBob = parent.Children.Where(c => c.Name == "Bob"); 
            // do something with the children
        }
    }
}

I know it's not the best example because in this case one would probably just query the Child entities directly, but I have encountered situations where I already had a Parent object and needed to traverse specific sub-trees through it.

Answer 1

Short answer: no. Longer answer: you can make it do this, with some sleight of hand.

Rippo's answer above shows how you would do it the 'proper' NHibernate way (whether it's with Linq or QueryOver or HQL doesn't really matter - the point is you have to step outside the parent -> child relationship to do a query). You can take this a step further and disguise this behind a façade. But to do so, you have to remove the mapped relationship entirely and replace it with a query at all times. You'd take out the Parent -> Children mapping, but leave the Child -> Parent mapping intact; then re-write the property on Parent to look like this:

public virtual IQueryable<Child> Children
{
    get
    {
        // somehow get a reference to the ISession (I use ambient context), then
        return session.Query<Child>().Where(c => c.Parent == this);
    }
}

Now, when you use Parent.Children you get back a queryable collection, so you could then write

IEnumerable<Child> childrenNamedBob = parent.Children.Where(c => c.Name == "Bob");

The only way you could do this and preserve the mapping is to amend NHibernate's collection objects (or inject your own). Diego Mijelshon (who is around these parts) wrote a spike of exactly that, adding IQueryable support to NHibernate collections so you could do

IEnumerable<Child> childrenNamedBob = parent.Children.AsQueryable().Where(c => c.Name == "Bob");

But from what I can see, this never went any further and there's no apparent plan to add this capability to NH. I have run Diego's code and it does work, but obviously it's not production quality and hasn't been tested, and I don't think it's ever been officially 'released' even as a private patch.

Here's the link to the discussion on the NH issue tracker: https://nhibernate.jira.com/browse/NH-2319

I believe NH should support this out of the box, as it's a natural way for most .NET devs to want to interact with pretty much anything enumerable, now that we have Linq, and not being able to do it without the side-effect of loading an unbounded collection into RAM sucks. But the traditional NH model is session -> query and that's what 99% of people use.

Answer 2

I asked the same question on NHusers a few weeks ago and didn't get an answer so I suspect the answer is you will always get all the parents children and then perform a in-memory filter. In many cases this might be the correct way in seeing it.

In your case I would rewrite the query to be:-

var childrenNamedBob = session.Query<Children>()
  .Where(w => w.Parent.Id == parentId && w.Name == "Bob");

Then simply to get parent (if childrenNamedBob has results) you could call:-

   var parent = childrenNamedBob.First().Parent;

or as you rightly pointed out:-

var parent = session.Get<Parent>(parentId);

Answer 3

You can now do that with NHibernate 5 directly without specific code !

See https://github.com/nhibernate/nhibernate-core/blob/master/releasenotes.txt

Build 5.0.0
=============================

** Highlights
...
    * Entities collections can be queried with .AsQueryable() Linq extension without being fully loaded.
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