BigBinary Blog

Override automatic updated_at in ActiveRecord/Rails

Rails provides some good tools like automatically updating created_at and updated_at columns. Developers do not need to worry about these columns. Rails updates these columns automatically which is great.

However I have a unique business need where I need to update a column but I do not want updated_at to be changed. Or we can see the problem this way. I want to change the updated_at to a particular value.

1>> User.first.update_attributes(:updated_at => 100.years.ago)
2UPDATE `users` SET `updated_at` = '2009-01-20 19:15:25' WHERE `id` = 2

Look at the sql that is generated. Rails discarded the updated_at value that I had supplied and replaced the value by the current time. Rails works fine if you supply created_at value. It is the updated_at value that is discarded.

Rails provides a feature called ActiveRecord::Base.record_timestamps . Using this feature I can tell rails to not to auto time stamp records.

Let's try that.

1>> User.record_timestamps=false
2=> false
3>> User.first.update_attributes(:updated_at => 100.years.ago)
4UPDATE `users` SET `updated_at` = '1909-01-20 18:52:50' WHERE `id` = 2
5>> User.record_timestamps=true
6=> true

It worked. I have successfully set updated_at to year 1909. However there is a problem.

For a brief duration User.record_timestamps was set to false. That is a class level variable. It means that for that brief duration if any other User record is updated then that record will not have correct updated_at value. That is not right. I want just one record ( User.first) to not to change updated_at without changing the behavior for the whole application.

In order to isolate the behavior to only the record we are interested in, I can do this.

1>> u = User.first
2
3>> class << u
4>>  def record_timestamps
5>>    false
6>>  end
7>> end
8
9>> u.update_attributes(:updated_at => 100.years.ago)
10UPDATE `users` SET `updated_at` = '1909-01-20 18:58:10' WHERE `id` = 2
11
12>> class << u
13>>  def record_timestamps
14>>    super
15>>  end
16>> end
17
18>> u.update_attributes(:updated_at => 200.years.ago)
19UPDATE `users` SET `updated_at` = '2009-01-20 19:22:11' WHERE `id` = 2

In order to restrict the changes to a model, I am opening up the metaclass of u ( user object) and in that object I am adding a method called record_timestamps . The idea is to insert a method called record_timestamps in the metaclass which will return true and in this way the changes are restricted to a single object rather than making change at the class level.

At this point the meta class of the user object has the method record_timestamps and this returns false. Now I update the record with updated_at set to 100 years ago. And I succeed.

Now I need to put the object behavior back to normal. I open up the metaclass and call super on the method so that the method call will go up the chain. And that's what happens when I try to test updated_at. This time the updated_at value that I set is ignored and rails changes the updated_at value.

update_without_timestamping method

This strategy of opening up an instance object works but it is messy. I would like to have a method that is much easier to use and this is what I came up with. Stick this piece of code in an initializer.

1module ActiveRecord
2  class Base
3
4    def update_record_without_timestamping
5      class << self
6        def record_timestamps; false; end
7      end
8
9      save!
10
11      class << self
12        def record_timestamps; super ; end
13      end
14    end
15
16  end
17end

This is how you can use it.

1>> u = User.first
2>> u.updated_at = 100.years.ago
3>> u.created_at = 200.years.ago
4>> u.update_record_without_timestamping
5UPDATE `users` SET `created_at` = '1809-01-20 19:08:21',
6`updated_at` = '1909-01-20 19:08:22' WHERE `id` = 2

Good usage of remove_method

In the above solution I used super when I want to bring back the default auto time stamping behavior. In stead of super I can also use remove_method. More about the what remove_method does is here .

1module ActiveRecord
2  class Base
3
4    def update_record_without_timestamping
5      class << self
6        def record_timestamps; false; end
7      end
8
9      save!
10
11      class << self
12        remove_method :record_timestamps
13      end
14    end
15
16  end
17end

Using the above technique, I can fully control updated_at values without rails messing up anything.