Rails provides some good tools like automatically updating
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
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.