validate at least one in has_and_belongs_to_many

Question

I have a model with:

has_and_belongs_to_many :users

How do I validate that the model has at least one user in the model? I tried:

Answer 1

In Rails 3.2.x:

validate :has_users?

def has_users?
  errors.add(:base, 'Error message') if self.users.blank?
end