Doctrine 2 - Optimal structure design for polymorphic relations

I'm working on a framework on which I wish to integrate Doctrine.

The already established structure works this way:

• Products table:

  • productId (PK)

• Categories table:

  • categoryId (PK)

etc. for other entities.

Where it gets complex is that, in order to introduce a "Multi-Store" concept, the framework introduces an additional table:

• ObjectSharing table:
  • ObjectId (PK)
  • ObjectType (PK) -> ('product', 'category', etc.)
  • StoreId (PK)

With a composite primary key on the three columns of the table.

How can I model this in Doctrine? I have tried multiple options:

• Class table inheritance (issue with the three primary keys, and the fact that a discriminator cannot be part of a primary key)
• Many-to-one relationships from ObjectSharing, with an abstract SharedObject entity from which I override the ids in the submethods, but I still get issues

Thanks for your help.