Fix adding/removing foreign keys on MySQL

db/migrate/20170526185602_add_stage_id_to_ci_builds.rb

@@ -3,15 +3,11 @@ class AddStageIdToCiBuilds < ActiveRecord::Migration
 
   DOWNTIME = false
 
-  disable_ddl_transaction!
-
   def up
     add_column :ci_builds, :stage_id, :integer
-    add_concurrent_foreign_key :ci_builds, :ci_stages, column: :stage_id, on_delete: :cascade
   end
 
   def down
-    remove_foreign_key :ci_builds, column: :stage_id
     remove_column :ci_builds, :stage_id, :integer
   end
 end

db/post_migrate/20170526185901_remove_stage_id_index_from_builds.rb

@@ -7,6 +7,7 @@ class RemoveStageIdIndexFromBuilds < ActiveRecord::Migration
 
   def up
     if index_exists?(:ci_builds, :stage_id)
+      remove_foreign_key(:ci_builds, column: :stage_id)
       remove_concurrent_index(:ci_builds, :stage_id)
     end
   end

db/post_migrate/20170621102400_add_stage_id_index_to_builds.rb

@@ -7,12 +7,14 @@ class AddStageIdIndexToBuilds < ActiveRecord::Migration
 
   def up
     unless index_exists?(:ci_builds, :stage_id)
+      add_concurrent_foreign_key(:ci_builds, :ci_stages, column: :stage_id, on_delete: :cascade)
       add_concurrent_index(:ci_builds, :stage_id)
     end
   end
 
   def down
     if index_exists?(:ci_builds, :stage_id)
+      remove_foreign_key(:ci_builds, column: :stage_id)
       remove_concurrent_index(:ci_builds, :stage_id)
     end
   end