Fix multiple prometheus alerts migration

db/schema.rb

@@ -2271,7 +2271,7 @@ ActiveRecord::Schema.define(version: 20180831164910) do
 
   add_index "prometheus_alerts", ["environment_id"], name: "index_prometheus_alerts_on_environment_id", using: :btree
   add_index "prometheus_alerts", ["project_id", "prometheus_metric_id"], name: "index_prometheus_alerts_on_project_id_and_prometheus_metric_id", unique: true, using: :btree
-  add_index "prometheus_alerts", ["prometheus_metric_id"], name: "index_prometheus_alerts_on_prometheus_metric_id_non_unique", using: :btree
+  add_index "prometheus_alerts", ["prometheus_metric_id"], name: "index_prometheus_alerts_on_prometheus_metric_id", using: :btree
 
   create_table "prometheus_metrics", force: :cascade do |t|
     t.integer "project_id"

ee/db/migrate/20180831134049_allow_many_prometheus_alerts.rb

@@ -9,17 +9,17 @@ class AllowManyPrometheusAlerts < ActiveRecord::Migration
   # and convert it into project+prometheus_metric unique
   def up
     rebuild_foreign_key do
-      add_concurrent_index :prometheus_alerts, [:project_id, :prometheus_metric_id], unique: true
-      add_concurrent_index :prometheus_alerts, :prometheus_metric_id, name: :index_prometheus_alerts_on_prometheus_metric_id_non_unique
       remove_concurrent_index :prometheus_alerts, :prometheus_metric_id, unique: true
+      add_concurrent_index :prometheus_alerts, :prometheus_metric_id
+      add_concurrent_index :prometheus_alerts, [:project_id, :prometheus_metric_id], unique: true
     end
   end
 
   def down
     rebuild_foreign_key do
-      add_concurrent_index :prometheus_alerts, :prometheus_metric_id, unique: true
       remove_concurrent_index :prometheus_alerts, [:project_id, :prometheus_metric_id], unique: true
-      remove_concurrent_index :prometheus_alerts, :prometheus_metric_id, name: :index_prometheus_alerts_on_prometheus_metric_id_non_unique
+      remove_concurrent_index :prometheus_alerts, :prometheus_metric_id
+      add_concurrent_index :prometheus_alerts, :prometheus_metric_id, unique: true
     end
   end
 
@@ -29,6 +29,7 @@ class AllowManyPrometheusAlerts < ActiveRecord::Migration
   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
@@ -36,6 +37,8 @@ class AllowManyPrometheusAlerts < ActiveRecord::Migration
     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
 end