Set container_expiration_policy for project first

This means we don't need the without_container_expiration_policy trait
anymore (which was globally modifying callbacks)

Add comment explaining why we need this and also comment on how
developers can assign existing projects.

spec/factories/container_expiration_policies.rb

@@ -2,7 +2,18 @@
 
 FactoryBot.define do
   factory :container_expiration_policy, class: 'ContainerExpirationPolicy' do
-    association :project, factory: [:project, :without_container_expiration_policy]
+    # Note: because of the project_id primary_key on
+    # container_expiration_policies, and the create_container_expiration_policy
+    # callback on Project, we need to build the project first before assigning
+    # it to a container_expiration_policy.
+    #
+    # Also, if you wish to assign an existing project to a
+    # container_expiration_policy, you will then have to destroy the project's
+    # container_expiration_policy first.
+    before(:create) do |container_expiration_policy|
+      container_expiration_policy.project = build(:project) unless container_expiration_policy.project
+    end
+
     cadence { '1d' }
     enabled { true }
 

spec/factories/projects.rb

@@ -140,12 +140,6 @@ FactoryBot.define do
       end
     end
 
-    trait :without_container_expiration_policy do
-      after(:build) do |project|
-        project.class.skip_callback(:create, :after, :create_container_expiration_policy, raise: false)
-      end
-    end
-
     # Build a custom repository by specifying a hash of `filename => content` in
     # the transient `files` attribute. Each file will be created in its own
     # commit, operating against the master branch. So, the following call:

spec/models/project_spec.rb

@@ -153,6 +153,16 @@ describe Project do
         expect(project.container_expiration_policy).to be_persisted
       end
 
+      it 'does not create another container expiration policy if there is already one' do
+        project = build(:project)
+
+        expect do
+          container_expiration_policy = create(:container_expiration_policy, project: project)
+
+          expect(project.container_expiration_policy).to eq(container_expiration_policy)
+        end.to change { ContainerExpirationPolicy.count }.by(1)
+      end
+
       it 'automatically creates a Pages metadata row' do
         expect(project.pages_metadatum).to be_an_instance_of(ProjectPagesMetadatum)
         expect(project.pages_metadatum).to be_persisted