ActiveRecord conditions of an association (Rails)

Question

Pretend I have a model, Post which has_many :comments. How can I only display posts that have comments?

I am somewhat comfortable with named_scope but I don't know how I can put Post.comments (or self.comments) in the :conditions hash which expects symbols.

class Post < ActiveRecord::Base
     has_many :comments
     named_scope :with_comments, :conditions => [#self.comments.length > 0]
end

What do I write in the commented area?

Thanks!

Answer 1

Better might be to put a counter_cache on Post.

class Comment < AR:Base
  belongs_to :post, :counter_cache => true
end

Then you only need to do 1 query instead of two.

Post.find(:all, :conditions => ["counter_cache > 0"])

Answer 2

You should be able to just join against your comments table, making sure to select the distinct rows

named_scope :with_comments, :joins => :comments, :select => 'DISTINCT posts.*'