ActiveRecord - querying polymorphic associations

Question

I am using polymorphic associations to track Comments in my project. All very straight forward stuff.

The problem I have is in querying based on the polymorphic association and joinging from the Comment model back to it's owner.

So ...

I have a Comment model

class Comment < ActiveRecord::Base
  belongs_to :commentable, :polymorphic => true
end

And a ForumTopics mode:

class ForumTopic < ActiveRecord::Base
  has_many :comments, :as => :commentable
end

I have several other "commentable" models that aren't important right now. All of this works.

What I am trying to do is find all of the Comments that belong to a ForumTopic with a specified condition (in this case, 'featured' == true).

When I try and use a finder to join the models:

@comments = Comment.find(:all 
            :joins => :commentable
            :conditions => ["forum_topics.featured = ? ", true] 
            )

I receive the following error:

Can not eagerly load the polymorphic association :commentable

Using the AR "include syntax":

@comments = Comment.find(:all 
            :include => :forum_topics
            :conditions => ["forum_topics.featured = ? ", true] 
            )

returns:

*Association named 'forum_topics' was not found; perhaps you misspelled it?*

If I try and join with a table name instead of the association name (string instead of symbol):

@comments = Comment.find(:all,
            :joins => "forum_topics",
            :conditions => ["forum_topics.featured = ? ", true] 
            )

I see:

Mysql::Error: Unknown table 'comments': SELECT comments. FROM comments forum_topics WHERE (forum_topics.featured = 1 )*

(You can see here that the syntax of the underlying query is totally off and the join is missing altogether).

Not sure if what I am doing is even possible, and there are other ways to achieve the required result but it seems like it should be doable.

Any ideas? Anything I am missing?

Answer 1

Argh!

I think I found the problem.

When joining via:

@comments = Comment.find(:all,
        :joins => "forum_topics",
        :conditions => ["forum_topics.featured = ? ", true] 
        )

You need the whole join!

:joins => "INNER JOIN forum_topics ON forum_topics.id = comments.commentable_id",

See the ever-awesome: http://guides.rubyonrails.org/active_record_querying.html#joining-tables

Answer 2

The accepted solution does not work once you introduce another model that has an association using "commentable". commentable_id is not unique and therefore you'll start retrieving the wrong comments.

For example:

You decide to add a news model that accepts comments...

class News < ActiveRecord::Base
   has_many :comments, :as => :commentable
end

Now you may get two records back if you made a comment on a forum_topic with an id of 1 and a news article with an id of 1 using your query:

:joins => "INNER JOIN forum_topics ON forum_topics.id = comments.commentable_id"

You could probably solve the problem by supplying a commentable_type as one of your conditions, but I don't think that's the best way to approach this issue.