Question

In my database there are four databases: Teachers, Students, Parents and Addresses.

Teachers, students, parents can have any number of addresses (zero or more).

I would like to use foreign keys to link between teachers and their addresses (and between students and their addresses etc).

Since records in Addresses can originate in either a teacher, a student or a parent, I cannot use a (single) foreign key in Addresses. And since Teachers etc can have any number of addresses, I cannot use an unlimited number of foreign keys in a Teacher record.

What would be a good strategy here?

I was thinking of using intermediate tables but that seems a bit cumbersome. Also, is it a good idea to have each Address record include 3 (NULLable) foreign keys, and only use one?

Answer 1

Something that has worked for me in the past is having a context and context_id field. Context would be the parent record (e.g. Teacher), and context_id would be its id. So, you’d have something like

teacher = find single teacher by id
addresses_for_teacher = find all addresses with context = 'teacher' and context_id = teacher.id

With appropriately indexed tables and a healthy serving of memcached we’re seeing great results on Cork’d.