Add cascade delete foreign key web_hooks, services

Some services create web_hooks but when deleted, they don't
cascade delete the web_hooks. So far this hasn't been a problem
because there is no UI for deleting services. However we might
add this functionality soon and need to make sure all dependent
data is deleted too.

changelogs/unreleased/283938-add-web_hooks-service-foreign-key.yml

+---
+title: Add cascade delete foreign key to web_hooks on service_id without validation
+merge_request: 47821
+author:
+type: fixed

db/migrate/20201119125730_add_web_hooks_service_foreign_key.rb

+# frozen_string_literal: true
+
+class AddWebHooksServiceForeignKey < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_web_hooks_on_service_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :web_hooks, :service_id, name: INDEX_NAME
+    add_concurrent_foreign_key :web_hooks, :services, column: :service_id, on_delete: :cascade, validate: false
+  end
+
+  def down
+    with_lock_retries do
+      remove_foreign_key_if_exists :web_hooks, column: :service_id
+    end
+
+    remove_concurrent_index_by_name :web_hooks, INDEX_NAME
+  end
+end

db/schema_migrations/20201119125730

+9fb3c338c999617b40e4c63f24ea037b5b1403354b93c2117e028ad8348d96aa
\ No newline at end of file

db/structure.sql

@@ -22455,6 +22455,8 @@ CREATE INDEX index_web_hooks_on_group_id ON web_hooks USING btree (group_id) WHE
 
 CREATE INDEX index_web_hooks_on_project_id ON web_hooks USING btree (project_id);
 
+CREATE INDEX index_web_hooks_on_service_id ON web_hooks USING btree (service_id);
+
 CREATE INDEX index_web_hooks_on_type ON web_hooks USING btree (type);
 
 CREATE UNIQUE INDEX index_webauthn_registrations_on_credential_xid ON webauthn_registrations USING btree (credential_xid);
@@ -23340,6 +23342,9 @@ ALTER TABLE ONLY environments
 ALTER TABLE ONLY ci_builds
     ADD CONSTRAINT fk_d3130c9a7f FOREIGN KEY (commit_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY web_hooks
+    ADD CONSTRAINT fk_d47999a98a FOREIGN KEY (service_id) REFERENCES services(id) ON DELETE CASCADE NOT VALID;
+
 ALTER TABLE ONLY ci_sources_pipelines
     ADD CONSTRAINT fk_d4e29af7d7 FOREIGN KEY (source_pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE;
 

spec/db/schema_spec.rb

@@ -86,7 +86,7 @@ RSpec.describe 'Database schema' do
     users_star_projects: %w[user_id],
     vulnerability_identifiers: %w[external_id],
     vulnerability_scanners: %w[external_id],
-    web_hooks: %w[service_id group_id]
+    web_hooks: %w[group_id]
   }.with_indifferent_access.freeze
 
   context 'for table' do