Add join table migration generator

For instance, running

    rails g migration CreateMediaJoinTable artists musics:uniq

will create a migration with

    create_join_table :artists, :musics do |t|
      # t.index [:artist_id, :music_id]
      t.index [:music_id, :artist_id], unique: true
    end

1 files changed, 17 insertions, 3 deletions

diff --git a/activerecord/test/cases/migration/create_join_table_test.rb b/activerecord/test/cases/migration/create_join_table_test.rb
index 8b91b3bc92..cd1b0e8b47 100644
--- a/activerecord/test/cases/migration/create_join_table_test.rb
+++ b/activerecord/test/cases/migration/create_join_table_test.rb
@@ -42,22 +42,36 @@ module ActiveRecord
       end
 
       def test_create_join_table_with_the_table_name
-        connection.create_join_table :artists, :musics, :table_name => :catalog
+        connection.create_join_table :artists, :musics, table_name: :catalog
 
         assert_equal %w(artist_id music_id), connection.columns(:catalog).map(&:name).sort
       end
 
       def test_create_join_table_with_the_table_name_as_string
-        connection.create_join_table :artists, :musics, :table_name => 'catalog'
+        connection.create_join_table :artists, :musics, table_name: 'catalog'
 
         assert_equal %w(artist_id music_id), connection.columns(:catalog).map(&:name).sort
       end
 
       def test_create_join_table_with_column_options
-        connection.create_join_table :artists, :musics, :column_options => {:null => true}
+        connection.create_join_table :artists, :musics, column_options: {null: true}
 
         assert_equal [true, true], connection.columns(:artists_musics).map(&:null)
       end
+
+      def test_create_join_table_without_indexes
+        connection.create_join_table :artists, :musics
+
+        assert connection.indexes(:artists_musics).blank?
+      end
+
+      def test_create_join_table_with_index
+        connection.create_join_table :artists, :musics do |t|
+          t.index [:artist_id, :music_id]
+        end
+
+        assert_equal [%w(artist_id music_id)], connection.indexes(:artists_musics).map(&:columns)
+      end
     end
   end
 end