I just ran across a weird glitch (IMHO) in find_or_create. The arguments passed to it are *not* added to the conditions for the 'first' part. This is odd, given that it's intended to replace find_or_create_by_* methods, which *did* use the specified values as conditions.
I'm unsure on whether this behavior is entirely undesirable, but it's definitely not what I had expected in my use case (ensuring that a join table record exists). Perhaps there should be a variant (find_or_create_exactly, perhaps?) that uses the supplied attributes as additional conditions.
If nothing else, the documentation should be updated to reflect this scenario - the last case in the examples *almost* describes this scenario, but the block form used there makes it unclear what:
User.where(:first_name => 'Scarlett').first_or_create(:last_name => "O'Hara")
I also found that DataMapper implements the behavior I was expecting in their first_or_create method: