ActiveRecord’s named_scopes are awesome, but I recently run into something less awesome about it: It doesn’t merge the :select option of nested scopes.

So let’s look something like that:

class Profile::Business < Profile::Profile
  # ... some code ...

  named_scope(:by_popularity, 
    :select => "profile_profiles.*, count(profile_fans.member_id) AS fan_count",
    # ... some more stuff for the scope
    :order => "fan_count DESC"
  )

  # ... some more code ...
end

And we use the named scope like this:

Profile::Business.by_popularity.find(:all, :select => 'profile_profile.*')

Should work, shouldn’t it? But as you might have guessed, it’s where ActiveRecord is exploding by throwing an ActiveRecord::StatementInvalid exception:

Mysql::Error: Unknown column 'fan_count' in 'order clause':

If you look at the rails log, you should see something like that:

Profile::Business Load (0.017810) SELECT profile_profiles.* FROM `profile_profiles` INNER JOIN [... some more SQL ...]

Yes, my first though was ‘Dude, where’s my SELECT?’. Turns out ActiveRecord doesn’t merge them it just use the last :select it encounters. In my case the workaround was easy by moving the COUNT() from :select to :order:

class Profile::Business < Profile::Profile
  # ... some code ...

  named_scope(:by_popularity, 
    :select => "profile_profiles.*",
    # ... some more stuff for the scope
    :order => "count(profile_fans.member_id) DESC"
  )

  # ... some more code ...
end

Crisis averted in this case, but it still leaves an uneasy feeling. And of course, the documentation says nothing about that.

Oh well… Someone proposed a patch for this problem and let’s hope it makes it into a Rails release.