Extract primary_key to AbstractReflection

[kamipo]

No description provided.

[matthewd]

Hmm.. I'm not sure what I think of this. On one hand, it avoids a duplicated method... but on the other, it adds that method to a bunch of classes for which it doesn't make sense. 🤔

[kamipo]

Should create new PR?

[kamipo]

I've created new PR #30209 and removed the commit that remove duplicated method in this PR.

[rafaelfranca]

I think if the method doesn't make sense in other classes we should not add it to the superclass even if it removed duplication. Unless it is a private utility method. If it is public, better to keep the duplication, or extract it to a module. In this case, the duplication is just fine in my opinion.

[kamipo]

Actually the primary_key is a private utility method that is used in association_primary_key and active_record_primary_key (by the way, association_primary_key has a bug #27609).
So I placed the method at bottom of private methods in AbstractReflection.