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

2 files changed, 7 insertions, 6 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 3c3f01223c..a246e6ace1 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -206,9 +206,12 @@ module ActiveRecord
         column_options = options.delete(:column_options) || {}
         column_options.reverse_merge!({:null => false})
 
+        t1_column, t2_column = [table_1, table_2].map{ |t| "#{t.to_s.singularize}_id" }
+
         create_table(join_table_name, options.merge!(:id => false)) do |td|
-          td.integer :"#{table_1.to_s.singularize}_id", column_options
-          td.integer :"#{table_2.to_s.singularize}_id", column_options
+          td.integer t1_column, column_options
+          td.integer t2_column, column_options
+          yield td if block_given?
         end
       end
 
diff --git a/activerecord/lib/active_record/migration/join_table.rb b/activerecord/lib/active_record/migration/join_table.rb
index 01a580781b..e880ae97bb 100644
--- a/activerecord/lib/active_record/migration/join_table.rb
+++ b/activerecord/lib/active_record/migration/join_table.rb
@@ -4,13 +4,11 @@ module ActiveRecord
       private
 
       def find_join_table_name(table_1, table_2, options = {})
-        options.delete(:table_name) { join_table_name(table_1, table_2) }
+        options.delete(:table_name) || join_table_name(table_1, table_2)
       end
 
       def join_table_name(table_1, table_2)
-        tables_names = [table_1, table_2].map(&:to_s).sort
-
-        tables_names.join("_").to_sym
+        [table_1, table_2].sort.join("_").to_sym
       end
     end
   end