When should I validate the presence of an ActiveRecord association object vs its id?

Question

Suppose I have an ActiveRecord association like:

class City < ActiveRecord::Base
  belongs_to :state
end

A city without a state should be invalid. It seems that both of these are possible validations:

validates :state,    presence: true
# OR
validates :state_id, presence: true

I would guess that they are identical, since:

• belongs_to creates methods state and state=
• state= sets the state_id

However, I've just fixed a failing spec by changing it to check for the id instead of the object.

Are these two ways of validating both acceptable? If so, when would you use one or the other?

Answer 1

validates :state will use the relationship from city to state (the belongs_to) along with the foreign key whereas validates :state_id alone will just use the column state_id and see if it has any value at all.

My preferred method is to validate state (the relationship) as this requires both the key and the relationship to be present.

Validating state_id will work, in that it will make sure that a state id exists, however it won't check for the validity of the code, i.e. that a state actually 'exists' for any given state key in City.

Basically if the foreign keys (for state_id) used in City all exist as actual records in State, the effect is the same. The difference would show if you had an invalid state code in state.

Answer 2

What if you did something like

s = State.new
c = City.new
c.state = s
c.valid?

I haven't tried this but I'm guessing that, if you're checking for the presence of c.state_id, it will be missing, even though c does have a state (because the ID hasn't been generated yet, because the state hasn't been saved yet).

That is to say, if what you care about is the presence of the state, you should validate the presence of the state.