Are foreign keys really necessary in a database design?

Question

As far as I know, foreign keys (FK) are used to aid the programmer to manipulate data in the correct way. Suppose a programmer is actually doing this in the right manner alr

Answer 1

I think some single thing at some point must be responsible for ensuring valid relationships.

For example, Ruby on Rails does not use foreign keys, but it validates all the relationships itself. If you only ever access your database from that Ruby on Rails application, this is fine.

However, if you have other clients which are writing to the database, then without foreign keys they need to implement their own validation. You then have two copies of the validation code which are most likely different, which any programmer should be able to tell is a cardinal sin.

At that point, foreign keys really are neccessary, as they allow you to move the responsibility to a single point again.