I used Rails 2.1’s named_scope to implement various ways to sort things on OsoEco. When I implemented “most discussed” on the Question model (questions have many comments), it involved joining in the comments table to count comments for each question. Initially, it looked something like:
named_scope :most_active, :joins => :comments, :group => "questions.id", order => "count(questions.id) desc"

That caused a problem, which the Pivotal Labs folks also commented on today:

When using named_scope, adding a :joins option will “mix-in” all of the attributes from that join table into your retrieved object, potentially overwriting any colliding attributes (including id … ouch!). There was consensus that this was a valuable feature, when used “properly”. Adding :select option can avoid this, or use :include.

Like they said, I fixed this with :select — the second try looked like this
named_scope :most_active, :select => "questions.*", :joins => :comments, :group => "questions.id", order => "count(questions.id) desc"

That worked (and fixed that problem), but it occurred to me that if my controller wanted to :include additional tables to add onto this scope (and that’s one of the cool things that named_scope enables), it wouldn’t work: Question.most_active.scoped(:include => :comments) raises a bad-SQL exception.

Fixing this required a bit of table aliasing, and led to this:
named_scope :most_active, :select => "questions.*", :joins => "left join comments as comments_for_count on comments_for_count.question_id = questions.id", :group => "questions.id", order => "count(questions.id) desc"

This worked, even with the :include added in a subsequent (anonymous) scope.