Fetching lazy loaded component causes n + 1 query when querying a subclass abstraction

[nfplee]

I've found that when doing a LINQ query and fetching a component it seems to work fine when querying a class, but if I try to query the interface the class implements then the fetch doesn't appear to work.

For example say I have the following:

public class Entity : IEntity
{
    public virtual int Id { get; set; 
    public virtual string Name { get; set; }
    public virtual Component Component { get; set; }
}

public class SubEntity : Entity, ISubEntity
{
    public virtual bool SomeProperty { get; set; }
}

public class Component
{
    public virtual string Field { get; set; }
}

public interface IEntity
{
    int Id { get; set; }
    string Name { get; set; }
    Component Component { get; set; }
}

public interface ISubEntity : IEntity
{
    public bool SomeProperty { get; set; }
}

Which is mapped like so:

protected override HbmMapping GetMappings()
{
    var mapper = new ModelMapper();
    mapper.Class<Entity>(rc =>
    {
        rc.Id(x => x.Id, m => m.Generator(Generators.Identity));
        rc.Property(x => x.Name);
        rc.Component(x => x.Component);
    });
    mapper.JoinedSubclass<SubEntity>(rc =>
    {
        rc.EntityName(typeof(ISubEntity).FullName);
        rc.Key(k => k.Column("Id"));
        rc.Property(x => x.SomeProperty);
    });
    mapper.Component<Component>(rc =>
    {
        rc.Property(x => x.Field);
        rc.Lazy(true);
    });

    return mapper.CompileMappingForAllExplicitlyAddedEntities();
}

Now if I was to say the following, the database would be hit once:

var result = session.Query<SubEntity>()
    .Fetch(e => e.Component)
    .ToList();

// Make sure the component field is executed.
foreach (var entity in result)
{
    var foo = entity.Component?.Field;
}

However if I say the following, the database would be hit for each entity in the result:

var result = session.Query<ISubEntity>()
    .Fetch(e => e.Component)
    .ToList();

// Make sure the component field is executed.
foreach (var entity in result)
{
    var foo = entity.Component?.Field;
}

Edit: See below for a test case.

[nfplee]

@bahusoid any chance you could have a look into this? I've tried myself by cloning the code and downloading the appropriate SDK. I've managed to replicate the issue but having stepped through the code I see in the Loader.cs (at which point the SQL has been generated) it includes the select for the lazy loaded property when querying over SubEntity (but not ISubEntity), however I can't see which file(s) convert the LINQ expression to the SQL.

Also I'm trying to produce a test case and while I can step over the code and see in the SQL profiler the issue, I can't see how to get a count of the total queries which are executed (even after turning on the statistics).

I'd appreciate any help as this is the first time I've dived into the NHibernate code and it's a bit alien to me.

It's also worth pointing out that if I use FetchLazyProperties then this problem doesn't happen.

[bahusoid]

any chance you could have a look into this?

Yeah, when I have time (not this month)

[nfplee]

After debugging I noticed the main different is the following line:

https://github.com/nhibernate/nhibernate-core/blob/master/src/NHibernate/Linq/Visitors/ResultOperatorProcessors/ProcessFetch.cs#L53

Returns null when resultOperator.RelationMember.ReflectedType is IEntity.

However, it returns the appropriate metadata when the resultOperator.RelationMember.ReflectedType is Entity.

[nfplee]

Here's a test case:

GH3289.zip

[bahusoid]

Fixed by #3320