Say you wanted to select all the accounts from your applications with no associated users. You’d think this would be obvious, however I racked my brain for a few minutes thinking of a Rails-way of sorting this out.

The good news is with Rails 2.2 or later, it can be accomplished like this thanks to join hashes.

Company.find(:all, :include => :users,
             :conditions => { :users => { :id => nil } })

Before Rails 2.2, you would have had to resort to the following.

Company.find(:all, :include => :users,
             :conditions => "users.id IS NULL")