Use up and down

db/migrate/20140122112253_create_merge_request_diffs.rb

 class CreateMergeRequestDiffs < ActiveRecord::Migration
-  def change
+  def up
     create_table :merge_request_diffs do |t|
       t.string :state, null: false, default: 'collected'
       t.text :st_commits, null: true
@@ -14,4 +14,8 @@ class CreateMergeRequestDiffs < ActiveRecord::Migration
       change_column :merge_request_diffs, :st_diffs, :text, limit: 2147483647
     end
   end
+
+  def down
+    drop_table :merge_request_diffs
+  end
 end