How do I test validity of a model object only on update using rspec?
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I have a profile model like so;

    class Profile < ActiveRecord::Base
      attr_accessible :first_name, :last_name
      belongs_to :user
      validates :user, presence: true
      validates :first_name, presence: true, on: :update
      validates :last_name, presence: true, on: :update
    end

I am wanting to write some rspec tests to test the validation of first_name and last_name but I cannot see how to run `profile.should_not be_valid` only on update in a model test. Something like;

    it "should be invalid without a first name on update" do
      profile = FactoryGirl.build :profile
      profile.first_name = nil
      profile_should_not be_valid
    end

doesn't differentiate between an update or create action and I can't see anything in the rspec documentation about this. Surely it's a fairly common thing to test.

