# With a subselect
@authors = Author.find(:all, :conditions => %{id in (
  select e.author_id from extensions e where e.size != 0
)})

# With the :joins key
@authors = Author.find(:all, {
  :joins => 'extensions on extensions.author_id = authors.id',
  :conditions => 'extensions.size != 0',
  :readonly => false # Not sure why :joins key implies ":readonly => true"
})