Force join for comparisons in hql when not null entity key can represent null entity

[bahusoid]

Fixes #1274 (criteria is not fixed, but criteria doesn't support implicit joins. So in criteria it's up to user to properly create joins for such associations)

Add force join for associations with EntityType.IsNullable == true (like not constrained one-to-one or associations with not-found="ignore" mapping):

nhibernate-core/src/NHibernate/Type/EntityType.cs

Lines 294 to 298 in 3d92bf4

	/// <summary>
	/// True if not null entity key can represent null entity
	/// (e.g. entity mapped with not-found="ignore" or not constrained one-to-one mapping)
	/// </summary>
	public abstract bool IsNullable { get; }

So when such property is used in comparisons like where alias.OneToOne is null we must add join and do comparison with joined table.

[hazzik]

Force pushed to resolve conflicts. The fixup commit got squashed.

[R0nd]

Can we get this reviewed?

[fredericDelaporte]

I agree for fixing the one-to-one non constrained case, where the foreign-key is (in true one-to-one cases) also the primary key and can never be null as such, and the the other side uses the foreign id generator, borrowing its primary key from the non-constrained entity.

But I disagree for the not-found ignore case, because it is detrimental for the general use-case, which is not to use this feature mainly meant for legacy DB with corrupted data. (Hibernate does not have this feature by the way, it is a NHibernate specific addition. I am not sure it is a good legacy to have added it.)

Adding joins for the general case, where foreign key are actual foreign keys, where a not-null foreign key can only mean the other side exists, is a bad move I think. So I think this PR should be adjusted to perform this change only for the one-to-one non-constrained case, and eventually for the case where a not-found ignore is detected.

[bahusoid]

Adding joins for the general case, where foreign key are actual foreign keys, where a not-null foreign key can only mean the other side exists, is a bad move

Just to be clear EntityType.IsNullable has nothing to do with not-null attribute. EntityType.IsNullable == true means not-null id can be treated as NULL entity for whatever reasons. And currently it's set for true for exactly two cases:

1. not constrained one-to-one
2. associations with not-found="ignore" mapping

nhibernate-core/src/NHibernate/Type/EntityType.cs

Lines 294 to 298 in 3d92bf4

	/// <summary>
	/// True if not null entity key can represent null entity
	/// (e.g. entity mapped with not-found="ignore" or not constrained one-to-one mapping)
	/// </summary>
	public abstract bool IsNullable { get; }

Offtopic: It's strange I didn't get notification for your review. Just noticed accidentally that PR was recently updated.