Incorrect metamodel for basic collections

It should be reopen since reverted by https://github.com/vladmihalcea/hibernate-orm/commit/df6d44f646d05f0fa3460887d0d05e9717031209

You are right that the spec does not cover this issue, but the spec does not take into consideration the use case when you’d want to map a List, Set, or Map to a JSON column in the database or when you want to map a List entity attribute to a PostgreSQL ARRAY column type.

Currently, this issue affects the Hibernate Types project as, even if the basic attribute is a List, it’s still a basic attribute, not a parent-child association that can be used in a JOIN.

Here’s the Hibernate Type issue for more context about the problem I’m talking about.

I will provide a PR to address this issue, and I’d appreciate it if you take a look at it.

I just added a note to the PR related to this fix and I'll amend this JIRA with the same:

I'm not convinced that the code added for this is necessarily spec compliant.

In JPA 2.1 section 3.8:

The attribute conversion facility allows the developer to specify methods to convert between the entity attribute representation and the database representation for attributes of basic types.

So when you have an entity which has a collection of basic types such as:

The attribute persistent type here is a `List<String>` regardless of the fact that it is stored in the database a single `String` column data type.

Also from the spec in section 6.2.1.1:

For every persistent collection-valued attribute z declared by class X, where the element type of z is Z, the metamodel class must contain a declaration as follows:

if the collection type of z is java.util.List,
then public static volatile ListAttribute<X, Z> z;

So I don't agree that this should be represented as a SingularAttribute as this PR does. I'm inclined to revert this fix in order to address HHH-12581.

The fix applied for this issue causes a regression when @ElementCollection and @CollectionTable is used. The following mapping taken from an entity used to work until Hibernate 5.2.16. It causes a NPE for criteria queries starting from 5.2.17.

Until 5.2.16, the JPA meta model class generated for this entity contains a SetAttribute for `readers`:

Using 5.2.17, this becomes a SingularAttribute instead, which is obviously wrong (and it seems that @ElementCollection is not taken into account).

As a result, the following code then throws a NPE when creating a fetch join (last line). Debugging showed that `Doc_.readers` is null.

Applied PR upstream.