Mark project_ci_cd_settings.merge_trains_enabled for removal

app/models/project_ci_cd_setting.rb

 # frozen_string_literal: true
 
 class ProjectCiCdSetting < ApplicationRecord
+  # TODO: remove once GitLab 12.7 is released
+  # https://gitlab.com/gitlab-org/gitlab/issues/36651
+  self.ignored_columns += %i[merge_trains_enabled]
   belongs_to :project, inverse_of: :ci_cd_settings
 
   # The version of the schema that first introduced this model/table.

ee/changelogs/unreleased/12796-remove-merge_trains_enabled-column-from-project_ci_cd_settings-t.yml

+---
+title: Ignore project_ci_cd_settings.merge_trains_enabled column
+merge_request: 19695
+author:
+type: added
+

ee/spec/services/merge_trains/create_pipeline_service_spec.rb

@@ -11,7 +11,7 @@ describe MergeTrains::CreatePipelineService do
   before do
     project.add_maintainer(maintainer)
     stub_licensed_features(merge_pipelines: true, merge_trains: true)
-    project.update!(merge_pipelines_enabled: true, merge_trains_enabled: true)
+    project.update!(merge_pipelines_enabled: true)
   end
 
   describe '#execute' do