Why doesn't updating this model trigger validations?

With Rails 3.1, I have:

class Status < ActiveRecord::Base
  has_many :participations, :dependent => :destroy
  has_many :users, :through => :participations

  validates :users, :presence => true
end

In my unit test, I do:

user = User.create(:name => "Bob")
status1 = Status.create(:description => "available")
user.statuses << status1

and get:

ActiveRecord::RecordInvalid: Validation failed: users can't be blank

but, if instead of the last line I do:

status1.users << user

it works fine.

Why does the validation get triggered for:

user.statuses << status1

UPDATE:

the test in full

 test "Return all statuses associated with the manager" do
    manager = Manager.create(:email => "foo@bar.com", :password => "password", :password_confirmation => "password")

    workshop = Workshop.create(:name => "Bob Autos")
    manager.workshop = workshop
    manager.save

    user = User.create(:name => "a customer")

    workshop.users << user

    sta开发者_运维知识库tus1 = Status.create(:description => "Car is ready")
    status2 = Status.create(:description => "problem with exhaust")
    status3 = Status.create(:description => "parts delivered")

    user.statuses << status1
    user.statuses << status2

    assert_equal([status1, status2], manager.statuses)

    user.statuses << status3

    assert_equal([status1, status2, status3], manager.statuses)
  end

The validation is on Status, not on User. A model's validation is only triggered when that model is updated, and you didn't actually update the User model (since statuses is an association, not a field).

If you want validation on one object to trigger validation on another, you should use validates_associated:

class Status
  validates_associated :users
end

(On a related note, though, why does a Status have many users? Seems like a Status should only be on one user.)

Because the status1.users collection isn't automatically updated when you do user.statuses << status1