Rspec rails on rails validation

 validate :cannot_modify_if_locked, on: :update


 def cannot_modify_if_locked
if self.locked
  errors.add(:locked_at, "#{self.locked_at} and cannot be modified")
  false
end

RSPEC

 consent_form = build(:consent_form, { locked_at: Date.today })
  expect(consent_form.).to eq true

This test is not good. how to write this test? i need to check with rspec this validation on update