Extract DatabaseTypeResolver module from Rails/BulkChangeTable cop

It could be reused in similar cases like #952.

lib/rubocop/cop/mixin/database_type_resolver.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    # A mixin to extend cops in order to determine the database type.
+    #
+    # This module automatically detect an adapter from `development` environment
+    # in `config/database.yml` or the environment variable `DATABASE_URL`
+    # when the `Database` option is not set.
+    module DatabaseTypeResolver
+      MYSQL = 'mysql'
+      POSTGRESQL = 'postgresql'
+
+      def database
+        cop_config['Database'] || database_from_yaml || database_from_env
+      end
+
+      private
+
+      def database_from_yaml
+        return unless database_yaml
+
+        case database_adapter
+        when 'mysql2'
+          MYSQL
+        when 'postgresql'
+          POSTGRESQL
+        end
+      end
+
+      def database_from_env
+        url = ENV['DATABASE_URL'].presence
+        return unless url
+
+        case url
+        when %r{\Amysql2://}
+          MYSQL
+        when %r{\Apostgres(ql)?://}
+          POSTGRESQL
+        end
+      end
+
+      def database_yaml
+        return unless File.exist?('config/database.yml')
+
+        yaml = if YAML.respond_to?(:unsafe_load_file)
+                 YAML.unsafe_load_file('config/database.yml')
+               else
+                 YAML.load_file('config/database.yml')
+               end
+        return unless yaml.is_a? Hash
+
+        config = yaml['development']
+        return unless config.is_a?(Hash)
+
+        config
+      rescue Psych::SyntaxError
+        # noop
+      end
+
+      def database_adapter
+        database_yaml['adapter'] || database_yaml.first.last['adapter']
+      end
+    end
+  end
+end

lib/rubocop/cop/rails/bulk_change_table.rb

@@ -64,16 +64,15 @@ module Rails
       #     end
       #   end
       class BulkChangeTable < Base
+        include DatabaseTypeResolver
+
         MSG_FOR_CHANGE_TABLE = <<~MSG.chomp
           You can combine alter queries using `bulk: true` options.
         MSG
         MSG_FOR_ALTER_METHODS = <<~MSG.chomp
           You can use `change_table :%<table>s, bulk: true` to combine alter queries.
         MSG
 
-        MYSQL = 'mysql'
-        POSTGRESQL = 'postgresql'
-
         MIGRATION_METHODS = %i[change up down].freeze
 
         COMBINABLE_TRANSFORMATIONS = %i[
@@ -175,55 +174,6 @@ def include_bulk_options?(node)
           options.hash_type? && options.keys.any? { |key| key.sym_type? && key.value == :bulk }
         end
 
-        def database
-          cop_config['Database'] || database_from_yaml || database_from_env
-        end
-
-        def database_from_yaml
-          return nil unless database_yaml
-
-          case database_adapter
-          when 'mysql2'
-            MYSQL
-          when 'postgresql'
-            POSTGRESQL
-          end
-        end
-
-        def database_adapter
-          database_yaml['adapter'] || database_yaml.first.last['adapter']
-        end
-
-        def database_yaml
-          return nil unless File.exist?('config/database.yml')
-
-          yaml = if YAML.respond_to?(:unsafe_load_file)
-                   YAML.unsafe_load_file('config/database.yml')
-                 else
-                   YAML.load_file('config/database.yml')
-                 end
-          return nil unless yaml.is_a? Hash
-
-          config = yaml['development']
-          return nil unless config.is_a?(Hash)
-
-          config
-        rescue Psych::SyntaxError
-          nil
-        end
-
-        def database_from_env
-          url = ENV['DATABASE_URL'].presence
-          return nil unless url
-
-          case url
-          when %r{\Amysql2://}
-            MYSQL
-          when %r{\Apostgres(ql)?://}
-            POSTGRESQL
-          end
-        end
-
         def support_bulk_alter?
           case database
           when MYSQL

lib/rubocop/cop/rails_cops.rb

@@ -3,6 +3,7 @@
 require_relative 'mixin/active_record_helper'
 require_relative 'mixin/active_record_migrations_helper'
 require_relative 'mixin/class_send_node_helper'
+require_relative 'mixin/database_type_resolver'
 require_relative 'mixin/enforce_superclass'
 require_relative 'mixin/index_method'
 require_relative 'mixin/migrations_helper'