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. Click here to find out how rails does auto time stamping.
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.
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.
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.
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.
This is how you can use it.
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 .
Using the above technique, I can fully control
updated_at values without rails messing up anything.