Update foreign key constraint for personal access tokens

Previously the foreign key constraint for `user_id` on the
`personal_access_tokens` table did not have the ON DELETE CASCADE
option enabled, which made deleting users that had generated a
PAT impossible.

This migration adds a new foreign key constraint with this option and
drops the old one.

Closes https://gitlab.com/gitlab-org/gitlab/issues/191262

changelogs/unreleased/sh-update-foreign-key-personal-access-tokens.yml

+---
+title: Update foreign key constraint for personal access tokens
+merge_request: 22305
+author:
+type: fixed

db/migrate/20191227140254_update_personal_access_tokens_user_id_foreign_key.rb

+# frozen_string_literal: true
+
+class UpdatePersonalAccessTokensUserIdForeignKey < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  CONSTRAINT_NAME = 'fk_personal_access_tokens_user_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_foreign_key(:personal_access_tokens, :users, column: :user_id, on_delete: :cascade, name: CONSTRAINT_NAME)
+    remove_foreign_key_if_exists(:personal_access_tokens, column: :user_id, on_delete: nil)
+  end
+
+  def down
+    add_concurrent_foreign_key(:personal_access_tokens, :users, column: :user_id, on_delete: nil)
+    remove_foreign_key_if_exists(:personal_access_tokens, column: :user_id, on_delete: :cascade, name: CONSTRAINT_NAME)
+  end
+end

db/schema.rb

@@ -4687,7 +4687,7 @@ ActiveRecord::Schema.define(version: 2019_12_29_140154) do
   add_foreign_key "pages_domains", "projects", name: "fk_ea2f6dfc6f", on_delete: :cascade
   add_foreign_key "path_locks", "projects", name: "fk_5265c98f24", on_delete: :cascade
   add_foreign_key "path_locks", "users"
-  add_foreign_key "personal_access_tokens", "users"
+  add_foreign_key "personal_access_tokens", "users", name: "fk_personal_access_tokens_user_id", on_delete: :cascade
   add_foreign_key "plan_limits", "plans", on_delete: :cascade
   add_foreign_key "pool_repositories", "projects", column: "source_project_id", on_delete: :nullify
   add_foreign_key "pool_repositories", "shards", on_delete: :restrict