Michael Hartl following limit >= 3

I'm following Michael Hartl tutorial and want to set a method to limit the amount of users a person can follow

Should I set the limit in the controller/relatiionships_controller.rb

app/model/relationships_controller.rb

validate :following_quota, :on => :create

private

def following_quota
 if user.active_relationships.size >= 3
  error.add(:base, 'exceeded follow limit')
 end
end

or should i set this is the model/user.rb