Rails: How to limit number of items in has_many association (from Parent)

Question

I would like to limit the number of items in an association. I want to ensure the User doesn\'t have more than X Things. This question was asked before and the solution had th

Answer 1

Validating on the current count causes the count to become greater than the limit once the save has completed. The only way I've found way to prevent the create from occurring is to validate that before the create, the number of things is less than the limit.

This is not to say that it isn't useful having a validation on the count in the User model, but doing so doesn't prevent User.things.create from being called because the user's count collection is valid until the new Thing object is saved, and then becomes invalid after the save.

class User
  has_many :things
end

class Thing
  belongs_to :user
  validate :on => :create do
    if user && user.things.length >= thing_limit
      errors.add(:user, :too_many_things)
    end
  end
end