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:

validates_presence_of :users

But that doesn't seem to give me what I want...

Answer 1

Josh Susser wrote a plugin that adds a validates_existence_of method that does what you want. It ensures that a foreign key references a record that exists.

Answer 2

I would write custom validation:

validate :has_users?

def has_users?
  errors.add_to_base "Model must have some users." if self.users.blank?
end

That would do exactly that.