Fix prometheus_metric_id non-unique migration

db/schema.rb

@@ -2270,7 +2270,8 @@ ActiveRecord::Schema.define(version: 20180831164910) do
   end
 
   add_index "prometheus_alerts", ["environment_id"], name: "index_prometheus_alerts_on_environment_id", using: :btree
-  add_index "prometheus_alerts", ["prometheus_metric_id"], name: "index_prometheus_alerts_on_prometheus_metric_id", unique: true, 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
 
   create_table "prometheus_metrics", force: :cascade do |t|
     t.integer "project_id"

ee/db/migrate/20180831134049_allow_many_prometheus_alerts.rb

@@ -5,13 +5,37 @@ class AllowManyPrometheusAlerts < ActiveRecord::Migration
 
   disable_ddl_transaction!
 
+  # We mutate `:prometheus_metric_id` into non unique one,
+  # and convert it into project+prometheus_metric unique
   def up
-    add_concurrent_index :prometheus_alerts, [:project_id, :prometheus_metric_id], unique: true
-    remove_concurrent_index :prometheus_alerts, :prometheus_metric_id, unique: true
+    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
+    end
   end
 
   def down
-    add_concurrent_index :prometheus_alerts, :prometheus_metric_id, unique: true
-    remove_concurrent_index :prometheus_alerts, [:project_id, :prometheus_metric_id], unique: true
+    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
+    end
+  end
+
+  private
+
+  # 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
+    end
+
+    yield
+
+    if Gitlab::Database.mysql?
+      add_concurrent_foreign_key :prometheus_alerts, :prometheus_metrics,
+        column: :prometheus_metric_id, on_delete: :cascade
+    end
   end
 end