QueryOver with Select and Fetch not working correctly

[mm-development]

I have found that sometimes fetching lazy properties doesn't work and property is still lazy. I've prepared a test code to reproduce this issue:

public class EntityA
    {
        public EntityA()
        {
            Items = new HashSet<EntityB>();
        }
        public virtual Guid Id { get; set; }
        
        public virtual string Name { get; set; }
        
        public virtual ICollection<EntityB> Items { get; set; }
    }

    public class EntityB
    {
        public virtual Guid Id { get; set; }
        
        public virtual string Name { get; set; }
    }

    public class EntityAMapping:ClassMapping<EntityA>
    {
        public EntityAMapping()
        {
            Id(x => x.Id, map => map.Generator(Generators.GuidComb));

            Property(x => x.Name, map =>
            {
                map.Length(256);
                map.NotNullable(true);
            });
            
            Set(x => x.Items, v =>
            {
                v.Cascade(Cascade.None);
                v.Key(x =>
                {
                    x.Column("RiskId");
                    x.NotNullable(true);
                });
                v.Lazy(CollectionLazy.Extra);
                v.Fetch(CollectionFetchMode.Subselect);
            }, h => h.ManyToMany(m => m.Column("RiskAreaId")));
        }
    }
    
    public class EntityBMapping:ClassMapping<EntityB>
    {
        public EntityBMapping()
        {
            Id(x => x.Id, map => map.Generator(Generators.GuidComb));

            Property(x => x.Name, map =>
            {
                map.Length(256);
                map.NotNullable(true);
            });
        }
    }
    
    [TestFixture]
    public class EmptyTest:NHibernateTestFixtureBase
    {
        [Test]
        public void Test()
        {
            using (var session = _sessionFactory.OpenSession())
            {
                var entityB = new EntityB();
                entityB.Name = "Name B";
                session.Save(entityB);
            
                var entityA = new EntityA();
                entityA.Name = "Name A";
                entityA.Items.Add(entityB);
                session.Save(entityA);
                session.Flush();
            }
            
            using (var session = _sessionFactory.OpenStatelessSession())
            {
                var obj=session.QueryOver<EntityA>().SingleOrDefault();
                Assert.IsFalse(NHibernateUtil.IsInitialized(obj.Items));
                
                obj=session.QueryOver<EntityA>().Fetch(SelectMode.Fetch,x=>x.Items).SingleOrDefault();
                Assert.IsTrue(NHibernateUtil.IsInitialized(obj.Items));
                
                obj=session.QueryOver<EntityA>().Fetch(SelectMode.Fetch,x=>x.Items)
                    .Select(Projections.RootEntity())
                    .SingleOrDefault();
                Assert.IsTrue(NHibernateUtil.IsInitialized(obj.Items)); //exception. Items not fetched!
                
                obj=session.QueryOver<EntityA>()
                    .Select(Projections.RootEntity().SetFetchLazyProperties())
                    .SingleOrDefault();
                Assert.IsTrue(NHibernateUtil.IsInitialized(obj.Items)); //exception. Items not fetched!
                
                obj=session.QueryOver<EntityA>().Fetch(SelectMode.Fetch,x=>x.Items)
                    .Select(Projections.RootEntity().SetFetchLazyProperties())
                    .SingleOrDefault();
                Assert.IsTrue(NHibernateUtil.IsInitialized(obj.Items)); //exception. Items not fetched!
            }
        }
    }

In this example if we have a QueryOver with a Select, then there is no way to eager fetch properties. Only way to force Items collection to be eager loaded is to change a mapping and set NoLazy instead of Extra.

If you need more info, please let me know

[bahusoid]

It's by design - projections completely overwrite SELECT statement. So all fetch requests won't be included in SELECT.

Use the following query to fetch root entity with lazy properties + Items collection:

session.QueryOver<EntityA>()
                    .Fetch(SelectMode.FetchLazyProperties, x => x)
                    .Fetch(SelectMode.Fetch, x => x.Items)
                    .TransformUsing(Transformers.DistinctRootEntity)
                    .SingleOrDefault();

[mm-development]

I can understand that this is by design but in this case, why eager loading from mapping works in Select? For me this is not very consistent behaviour. I would rather prefer opposite approach: if we overwrite Select that eager loading from mapping doesn't work (by design) but if you force eager in your query by Fetch() method that this fetch will work.
And about your query. Unfortunately this not helping me because my real query is much more complex. And I have to overwrite Select

[bahusoid]

why eager loading from mapping works in Select

Are you sure it's loaded in single query? I think your collection is loaded in separate query in this case.

Also just in case SetFetchLazyProperties and SelectMode.FetchLazyProperties, is about lazy scalar properties. References and collections are not fetched by it. I see no lazy scalar properties in your mappings.

[mm-development]

Are you sure it's loaded in single query? I think your collection is loaded in separate query in this case.

All fetches are loaded in separate SQL queries (query per object type). But from NHibernate user point of view this is done automatically and in one NHibernate query.

I see no lazy properties in your mappings.

As I mentioned, my test code is simple to show the problem I've encountered. In my real query I have many references and collections I need to fetch. To illustrate my real case: imaging that in my class Entity A I have a few collections like Items, my EntityB has also collection like Items and a few references and so on.
Now I'm trying to create a Select part and include all required objects and probably this is doable but I would be nice to use NHibnerate mechanisms to fetch additional data specified by Fetch().

[bahusoid]

why eager loading from mapping works in Select

Then answer is simple - it has nothing to do with Select. Mapping eager loading works for entity no matter how it's loaded via session.Get or via query with/without Select.

Fetches for projections are not supported.

[mm-development]

Ok, thanks for the answer