Use cascading delete for openid_request upon deleting an access_grant

This change adds `on_delete: :cascade`for the
foreign_key in `oauth_openid_requests` table.

changelogs/unreleased/23746-error-500-when-trying-to-destroy-oauth-application-entry-in-admin-a.yml

+---
+title: Use cascading deletes for deleting oauth_openid_requests upon deleting an oauth_access_grant
+merge_request: 19617
+author:
+type: fixed

db/migrate/20191107064946_update_oauth_open_id_requests_foreign_keys.rb

+# frozen_string_literal: true
+
+# rubocop: disable Migration/AddConcurrentForeignKey
+
+class UpdateOauthOpenIdRequestsForeignKeys < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    remove_foreign_key_if_exists(:oauth_openid_requests, column: :access_grant_id)
+    add_foreign_key(:oauth_openid_requests, :oauth_access_grants, column: :access_grant_id, on_delete: :cascade)
+  end
+
+  def down
+    remove_foreign_key_if_exists(:oauth_openid_requests, column: :access_grant_id)
+    add_foreign_key(:oauth_openid_requests, :oauth_access_grants, column: :access_grant_id, on_delete: false, name: 'fk_oauth_openid_requests_oauth_access_grants_access_grant_id')
+  end
+end

db/schema.rb

@@ -4532,7 +4532,7 @@ ActiveRecord::Schema.define(version: 2019_11_25_140458) do
   add_foreign_key "notes", "projects", name: "fk_99e097b079", on_delete: :cascade
   add_foreign_key "notes", "reviews", name: "fk_2e82291620", on_delete: :nullify
   add_foreign_key "notification_settings", "users", name: "fk_0c95e91db7", on_delete: :cascade
-  add_foreign_key "oauth_openid_requests", "oauth_access_grants", column: "access_grant_id", name: "fk_oauth_openid_requests_oauth_access_grants_access_grant_id"
+  add_foreign_key "oauth_openid_requests", "oauth_access_grants", column: "access_grant_id", name: "fk_77114b3b09", on_delete: :cascade
   add_foreign_key "operations_feature_flag_scopes", "operations_feature_flags", column: "feature_flag_id", on_delete: :cascade
   add_foreign_key "operations_feature_flags", "projects", on_delete: :cascade
   add_foreign_key "operations_feature_flags_clients", "projects", on_delete: :cascade

spec/factories/oauth_openid_requests.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :oauth_openid_request, class: 'Doorkeeper::OpenidConnect::Request' do
+    access_grant factory: :oauth_access_grant
+    sequence(:nonce) { |n| n.to_s }
+  end
+end

spec/models/oauth_access_grant_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe OauthAccessGrant do
+  let(:user) { create(:user) }
+  let(:application) { create(:oauth_application, owner: user) }
+
+  describe '#delete' do
+    it 'cascades to oauth_openid_requests' do
+      access_grant = create(:oauth_access_grant, application: application)
+      create(:oauth_openid_request, access_grant: access_grant)
+
+      expect { access_grant.delete }.to change(Doorkeeper::OpenidConnect::Request, :count).by(-1)
+    end
+  end
+end