Cleanup web_hooks service_id foreign key

This removes records from the web_hooks table
that belong to a service which doesn't exist
anymore. This is to validate the foreign key
constraint on web_hooks.service_id

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

+---
+title: Remove orphan service hooks
+merge_request: 48263
+author:
+type: fixed

db/post_migrate/20201203123201_remove_orphan_service_hooks.rb

+# frozen_string_literal: true
+
+class RemoveOrphanServiceHooks < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  class WebHook < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'web_hooks'
+
+    def self.service_hooks
+      where(type: 'ServiceHook')
+    end
+  end
+
+  class Service < ActiveRecord::Base
+    self.table_name = 'services'
+  end
+
+  def up
+    WebHook.service_hooks.where.not(service_id: Service.select(:id)).where.not(service_id: nil).each_batch do |relation|
+      relation.delete_all
+    end
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20201203123201

+85a642d60e92a880e0a0699f8dcca42aebe2b5363bfcc3010e647734c7cb7dec
\ No newline at end of file

spec/migrations/remove_orphan_service_hooks_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require_migration!
+require Rails.root.join('db', 'migrate', '20201119125730_add_web_hooks_service_foreign_key.rb')
+
+RSpec.describe RemoveOrphanServiceHooks, schema: 20201203123201 do
+  let(:web_hooks) { table(:web_hooks) }
+  let(:services) { table(:services) }
+
+  before do
+    services.create!
+    web_hooks.create!(service_id: services.first.id, type: 'ServiceHook')
+    web_hooks.create!(service_id: nil)
+
+    AddWebHooksServiceForeignKey.new.down
+    web_hooks.create!(service_id: non_existing_record_id, type: 'ServiceHook')
+    AddWebHooksServiceForeignKey.new.up
+  end
+
+  it 'removes service hooks where the referenced service does not exist', :aggregate_failures do
+    expect { RemoveOrphanServiceHooks.new.up }.to change { web_hooks.count }.by(-1)
+    expect(web_hooks.where.not(service_id: services.select(:id)).count).to eq(0)
+  end
+end