I agree, whether or not a given data model has foreign keys is a math question with a correct answer (up to isomorphism of the data model); if it does, where in

I agree, whether or not a given data model has foreign keys is a math question with a correct answer (up to isomorphism of the data model); if it does, where in a system they should or should not be enforced is an entirely separate, implementation (vs semantic) level issue.