adding invertable migration test

activerecord/test/cases/invertable_migration_test.rb

@@ -0,0 +1,42 @@
+require "cases/helper"
+
+module ActiveRecord
+  class InvertableMigrationTest < ActiveRecord::TestCase
+    class InvertableMigration < ActiveRecord::Migration
+      def change
+        create_table("horses") do |t|
+          t.column :content, :text
+          t.column :remind_at, :datetime
+        end
+      end
+
+      def write(text = '')
+        # sssshhhhh!!
+      end
+    end
+
+    def treardown
+      if ActiveRecord::Base.connection.table_exists?("horses")
+        ActiveRecord::Base.connection.drop_table("horses")
+      end
+    end
+
+    def test_invertable?
+      migration = InvertableMigration.new
+      assert migration.invertable?, 'should be invertable'
+    end
+
+    def test_up
+      migration = InvertableMigration.new
+      migration.migrate(:up)
+      assert migration.connection.table_exists?("horses"), "horses should exist"
+    end
+
+    def test_down
+      migration = InvertableMigration.new
+      migration.migrate :up
+      migration.migrate :down
+      assert !migration.connection.table_exists?("horses")
+    end
+  end
+end