add proper post_uploads reverse index

db/migrate/20130613211700_drop_posts_uploads.rb

@@ -0,0 +1,16 @@
+class DropPostsUploads < ActiveRecord::Migration
+  def up
+    drop_table :posts_uploads
+  end
+
+  def down
+    create_table :posts_uploads, id: false do |t|
+      t.integer :post_id
+      t.integer :upload_id
+  end
+
+    add_index :posts_uploads, :post_id
+    add_index :posts_uploads, :upload_id
+    add_index :posts_uploads, [:post_id, :upload_id], unique: true
+  end
+end

db/migrate/20130613212230_create_post_uploads.rb

@@ -0,0 +1,15 @@
+class CreatePostUploads < ActiveRecord::Migration
+  def up
+    create_table :post_uploads do |t|
+      t.integer :post_id, null: false
+      t.integer :upload_id, null: false
+    end
+
+    # no support for this till rails 4
+    execute 'create unique index idx_unique_post_uploads on post_uploads(post_id, upload_id)'
+  end
+
+  def down
+    drop_table :post_uploads
+  end
+end