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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.
>> User.first.update_attributes(:updated_at => 100.years.ago) UPDATE `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.
>> User.record_timestamps=false => false >> User.first.update_attributes(:updated_at => 100.years.ago) UPDATE `users` SET `updated_at` = '1909-01-20 18:52:50' WHERE `id` = 2 >> User.record_timestamps=true => 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.
>> u = User.first >> class << u >> def record_timestamps >> false >> end >> end >> u.update_attributes(:updated_at => 100.years.ago) UPDATE `users` SET `updated_at` = '1909-01-20 18:58:10' WHERE `id` = 2 >> class << u >> def record_timestamps >> super >> end >> end >> u.update_attributes(:updated_at => 200.years.ago) UPDATE `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.
module ActiveRecord class Base def update_record_without_timestamping class << self def record_timestamps; false; end end save! class << self def record_timestamps; super ; end end end end end
This is how you can use it.
>> u = User.first >> u.updated_at = 100.years.ago >> u.created_at = 200.years.ago >> u.update_record_without_timestamping UPDATE `users` SET `created_at` = '1809-01-20 19:08:21', `updated_at` = '1909-01-20 19:08:22' WHERE `id` = 2
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 .
module ActiveRecord class Base def update_record_without_timestamping class << self def record_timestamps; false; end end save! class << self remove_method :record_timestamps end end end end
Using the above technique, I can fully control
updated_at values without rails messing up anything.