database design - Defining multiple foreign keys in one table to many tables

Question

I have 3 models:

Post:

• id
• title
• body

Photo:

• id
• filepath

Comment:

• id
• post_id
• body

and corresponding tables in DB. Now, if I want to have comments only for my posts I can simply add following foreign key: ALTER TABLE comment ADD FOREIGN KEY (post_id) REFERENCES post (id). But I want to have comments for other models (photo, profile, video, etc) and keep all comments in one table. How can I define foreign keys (i definitely need FKs for ORM) in such case?

Answer 1

Find something common to post, profile, etc -- I have used Entity for a lack of better word, then subtype.

• In this model one entity can have many comments, one comment belongs to one entity only.