I have written previously about how much I like named_scopes in Rails and I still do very much. After using them for some time I got tripped up on an issue that came up with them which surprised me a bit. I thought it would be a good idea to document it here in case others have the same issue.
To demonstrate the issue, lets say I have an app that has a User model. On the home page I want to display a list of the users which have signed up in the last hour. This is an excellent use case for a named scope. We can start by creating a named scope called "recent" which will then allow us to simply say "User.recent" to retrieve all the recently created accounts from the database. This seems simple enough so I went ahead and wrote it up as follows:
Now you will notice that I named it recent_bad, and that is because this named scope is BAD! Take a look at the queries generated when I call recent_bad three times, notice anything wrong? Its subtle. Note how the date after created_at "2010-01-05 16:55:44" never changes. For effect I made the model acts_as_paranoid so you can see what the timestamp should be. What is happening here? The named_scope is at the class level, that means that the when the User class is loaded the Time.now.utc is evaluated once and then never again. This is why the time only changes when the server is restarted. In order to avoid this issue simply put the condition within a lambda as follows:
Now you will see that the created_at time is updating as it should be. Its a subtle bug and one that caught me by surprise.