"Agile Web Development with Rails" by Dave Thomas, page 322:
You can choose to to establish these [foreign-key] constraints if you
want (and I personally recommend that you do), but the foreign-key
support in Rails doesn't need them.
"The Rails Way" by Obie Fernandez, page 296:
... we haven't really touched on a subject of particular importance to
many programmers: foreign-key constraints in the database. That's
mainly because use of foreign-key constraints simply isn't the Rails
way to tackle the problem of relational integerity.
I know there won't be unanimous consensus on this topic, but is there
a "Rails Way" concerning foreign keys? If "The Way" is not to use
them, what do we use?