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, 27 insertions, 6 deletions

diff --git a/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb b/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb
index 1509e34473..c1f9d8962b 100644
--- a/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb
+++ b/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb
@@ -11,15 +11,28 @@ module ActiveRecord
       end
 
       protected
-        attr_reader :migration_action
+      attr_reader :migration_action, :join_tables
 
-        def set_local_assigns!
-          if file_name =~ /^(add|remove)_.*_(?:to|from)_(.*)/
-            @migration_action = $1
-            @table_name       = $2.pluralize
+      def set_local_assigns!
+        case file_name
+        when /^(add|remove)_.*_(?:to|from)_(.*)/
+          @migration_action = $1
+          @table_name       = $2.pluralize
+        when /join_table/
+          if attributes.length == 2
+            @migration_action = 'join'
+            @join_tables      = attributes.map(&:name)
+
+            set_index_names
           end
         end
+      end
 
+      def set_index_names
+        attributes.each_with_index do |attr, i|
+          attr.index_name = [attr, attributes[i - 1]].map{ |a| :"#{a.name.singularize}_id"}
+        end
+      end
     end
   end
 end
diff --git a/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb b/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb
index 34eaffcb0f..d5c07aecd3 100644
--- a/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb
+++ b/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb
@@ -12,6 +12,14 @@ class <%= migration_class_name %> < ActiveRecord::Migration
   <%- end -%>
 <%- end -%>
   end
+<%- elsif migration_action == 'join' -%>
+  def change
+    create_join_table :<%= join_tables.first %>, :<%= join_tables.second %> do |t|
+    <%- attributes.each do |attribute| -%>
+      <%= '# ' unless attribute.has_index? -%>t.index <%= attribute.index_name %><%= attribute.inject_index_options %>
+    <%- end -%>
+    end
+  end
 <%- else -%>
   def up
 <% attributes.each do |attribute| -%>
@@ -40,4 +48,4 @@ class <%= migration_class_name %> < ActiveRecord::Migration
 <%- end -%>
   end
 <%- end -%>
-end
\ No newline at end of file
+end