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!

score 2 · Answer 1 · answered Jun 16 '09 at 22:27

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.*'

answered Jun 16 '09 at 22:27

slillibri

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looks awesome. How can I pass in arguments allowing me to manually enter in attribute names. my posts table has a pk called postid and the comments table has an fk called post_id. I know how to manually set pks and fks in the associations, how do I do it with named_scope? thanks again, this really is a quality response. – user94154 Jun 17 '09 at 00:49
Just replace the :joins option with the join you want. You probably want something like the following, :joins => 'INNER JOIN "comments" ON comments.post_id = posts.postid' – slillibri Jun 17 '09 at 22:55
In Rails, `:joins` defaults to use `INNER JOIN` – Bart Jedrocha Oct 18 '11 at 01:35

score 1 · Accepted Answer · answered Jun 17 '09 at 03:53

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"])

answered Jun 17 '09 at 03:53

BJ Clark

3,215
2
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ActiveRecord conditions of an association (Rails)

2 Answers2