module ActiveModel::Lint::Tests

of the model, and is used to a generate unique DOM id for the object.
to_key returns an Enumerable of all (primary) key attributes

Fails otherwise.
this method returns +nil+ when the object is not persisted.
Passes if the object's model responds to to_key and if calling

def test_to_key
  assert_respond_to model, :to_key
  def model.persisted?() false end
  assert model.to_key.nil?, "to_key should return nil when `persisted?` returns false"
end