Rebuild foreign keys for MySQL to fix broken master

MySQL requires to drop FK for time of re-adding index

ee/db/migrate/20180912113336_allow_prometheus_alerts_per_environment.rb

 # frozen_string_literal: true
 
-class AllowPrometheusAlertsPerEnvironment < ActiveRecord::Migration
+class AllowPrometheusAlertsPerEnvironment < ActiveRecord::Migration[5.0]
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = false
@@ -9,6 +9,7 @@ class AllowPrometheusAlertsPerEnvironment < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
+    rebuild_foreign_key do
       # Before we create the new index we need to remove it to deal with possible
       # failures from previous migration.
       #
@@ -20,10 +21,12 @@ class AllowPrometheusAlertsPerEnvironment < ActiveRecord::Migration
 
       remove_concurrent_index :prometheus_alerts, old_columns
     end
+  end
 
   def down
     delete_duplicate_alerts!
 
+    rebuild_foreign_key do
       # Before we create the new index we need to remove it to deal with possible
       # failures from previous migration.
       #
@@ -35,6 +38,7 @@ class AllowPrometheusAlertsPerEnvironment < ActiveRecord::Migration
       remove_concurrent_index :prometheus_alerts, new_columns,
         name: INDEX_METRIC_ENVIRONMENT_NAME
     end
+  end
 
   private
 
@@ -58,6 +62,23 @@ class AllowPrometheusAlertsPerEnvironment < ActiveRecord::Migration
     end
   end
 
+  # MySQL requires to drop FK for time of re-adding index
+  def rebuild_foreign_key
+    if Gitlab::Database.mysql?
+      remove_foreign_key_without_error :prometheus_alerts, :prometheus_metrics
+      remove_foreign_key_without_error :prometheus_alerts, :projects
+    end
+
+    yield
+
+    if Gitlab::Database.mysql?
+      add_concurrent_foreign_key :prometheus_alerts, :prometheus_metrics,
+        column: :prometheus_metric_id, on_delete: :cascade
+      add_concurrent_foreign_key :prometheus_alerts, :projects,
+        column: :project_id, on_delete: :cascade
+    end
+  end
+
   def new_columns
     [:project_id, :prometheus_metric_id, :environment_id]
   end