Stub callback in factory instead of skipping it

Instead of skipping a callback, stub it on the instance, this avoids
messing up the factory for any future use.

See https://gitlab.com/gitlab-org/gitlab/-/issues/247865

ee/spec/models/geo/merge_request_diff_registry_spec.rb

@@ -3,7 +3,9 @@
 require 'spec_helper'
 
 RSpec.describe Geo::MergeRequestDiffRegistry, :geo, type: :model do
-  let_it_be(:registry) { create(:geo_merge_request_diff_registry) }
+  let_it_be(:merge_request) { create(:merge_request) }
+  let_it_be(:merge_request_diff) { create(:merge_request_diff, merge_request: merge_request) }
+  let_it_be(:registry) { create(:geo_merge_request_diff_registry, merge_request_diff: merge_request_diff) }
 
   specify 'factory is valid' do
     expect(registry).to be_valid

spec/factories/merge_request_diffs.rb

@@ -2,7 +2,13 @@
 
 FactoryBot.define do
   factory :merge_request_diff do
-    association :merge_request, :without_merge_request_diff
+    merge_request do
+      build(:merge_request) do |merge_request|
+        # MergeRequest should not create a MergeRequestDiff in the callback
+        allow(merge_request).to receive(:ensure_merge_request_diff)
+      end
+    end
+
     state { :collected }
     commits_count { 1 }
 

spec/factories/merge_requests.rb

@@ -92,16 +92,6 @@ FactoryBot.define do
       target_branch { "feature_two" }
     end
 
-    trait(:without_merge_request_diff) do
-      after(:build) do |_|
-        MergeRequest.skip_callback(:create, :after, :ensure_merge_request_diff)
-      end
-
-      after(:create) do |_|
-        MergeRequest.set_callback(:create, :after, :ensure_merge_request_diff)
-      end
-    end
-
     trait :locked do
       state_id { MergeRequest.available_states[:locked] }
     end