Enforce use of add_concurrent_foreign_key

This adds a Rubocop rule to enforce the use of
add_concurrent_foreign_key instead of the regular add_foreign_key
method. This cop has been disabled for existing migrations so we don't
need to change those.

db/migrate/20160919145149_add_group_id_to_labels.rb

@@ -7,7 +7,7 @@ class AddGroupIdToLabels < ActiveRecord::Migration
 
   def change
     add_column :labels, :group_id, :integer
-    add_foreign_key :labels, :namespaces, column: :group_id, on_delete: :cascade
+    add_foreign_key :labels, :namespaces, column: :group_id, on_delete: :cascade # rubocop: disable Migration/AddConcurrentForeignKey
     add_concurrent_index :labels, :group_id
   end
 end

db/migrate/20161020083353_add_pipeline_id_to_merge_request_metrics.rb

@@ -28,6 +28,6 @@ class AddPipelineIdToMergeRequestMetrics < ActiveRecord::Migration
   def change
     add_column :merge_request_metrics, :pipeline_id, :integer
     add_concurrent_index :merge_request_metrics, :pipeline_id
-    add_foreign_key :merge_request_metrics, :ci_commits, column: :pipeline_id, on_delete: :cascade
+    add_foreign_key :merge_request_metrics, :ci_commits, column: :pipeline_id, on_delete: :cascade # rubocop: disable Migration/AddConcurrentForeignKey
   end
 end

db/migrate/20161031171301_add_project_id_to_subscriptions.rb

@@ -5,7 +5,7 @@ class AddProjectIdToSubscriptions < ActiveRecord::Migration
 
   def up
     add_column :subscriptions, :project_id, :integer
-    add_foreign_key :subscriptions, :projects, column: :project_id, on_delete: :cascade
+    add_foreign_key :subscriptions, :projects, column: :project_id, on_delete: :cascade # rubocop: disable Migration/AddConcurrentForeignKey
   end
 
   def down

rubocop/cop/migration/add_concurrent_foreign_key.rb

+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if `add_concurrent_foreign_key` is used instead of
+      # `add_foreign_key`.
+      class AddConcurrentForeignKey < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = '`add_foreign_key` requires downtime, use `add_concurrent_foreign_key` instead'
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          name = node.children[1]
+
+          add_offense(node, :selector) if name == :add_foreign_key
+        end
+
+        def method_name(node)
+          node.children.first
+        end
+      end
+    end
+  end
+end

rubocop/rubocop.rb

 require_relative 'cop/gem_fetcher'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_column_with_default'
+require_relative 'cop/migration/add_concurrent_foreign_key'
 require_relative 'cop/migration/add_index'

spec/rubocop/cop/migration/add_concurrent_foreign_key_spec.rb

+require 'spec_helper'
+require 'rubocop'
+require 'rubocop/rspec/support'
+require_relative '../../../../rubocop/cop/migration/add_concurrent_foreign_key'
+
+describe RuboCop::Cop::Migration::AddConcurrentForeignKey do
+  include CopHelper
+
+  let(:cop) { described_class.new }
+
+  context 'outside of a migration' do
+    it 'does not register any offenses' do
+      inspect_source(cop, 'def up; add_foreign_key(:projects, :users, column: :user_id); end')
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'in a migration' do
+    before do
+      allow(cop).to receive(:in_migration?).and_return(true)
+    end
+
+    it 'registers an offense when using add_foreign_key' do
+      inspect_source(cop, 'def up; add_foreign_key(:projects, :users, column: :user_id); end')
+
+      aggregate_failures do
+        expect(cop.offenses.size).to eq(1)
+        expect(cop.offenses.map(&:line)).to eq([1])
+      end
+    end
+  end
+end