title: is there a way to eager load polymorphic association's associations?
body: <p>artists have many activities (basically a cache of interactions between users):</p>
  
  <pre><code>class Activity &lt; ActiveRecord::Base
  
    belongs_to :receiver, :class_name =&gt; 'Artist', :foreign_key =&gt; :receiver_id #owns the stuff done "TO" him
    belongs_to :link, :polymorphic =&gt; true
    belongs_to :creator, :class_name =&gt; 'Artist', :foreign_key =&gt; :creator_id #person who initiated the activity
  
  end
  </code></pre>
  
  <p>For example: </p>
  
  <pre><code>  Activity.create(:receiver_id =&gt; author_id, :creator_id =&gt; artist_id, :link_id =&gt; id, :link_type =&gt; 'ArtRating') 
  </code></pre>
  
  <p>I want to create an activity stream page for each artist, consisting of a list of different types of events, ArtRatings (likes, dislikes), Favoriting, Following etc.</p>
  
  <p>The controller looks like this:</p>
  
  <pre><code>class ActivityStreamController &lt; ApplicationController
    def index
      @activities = @artist.activities.includes([:link,:creator,:receiver]).order("id DESC").limit(30)
    end
  end
  </code></pre>
  
  <p>The db call correctly eagerly loads the polymorphic link objects:</p>
  
  <pre><code>  SELECT "activities".* FROM "activities" WHERE (("activities"."receiver_id" = 6 OR "activities"."creator_id" = 6)) ORDER BY id DESC LIMIT 30
    ArtRating Load (0.5ms)  SELECT "art_ratings".* FROM "art_ratings" WHERE "art_ratings"."id" IN (137, 136, 133, 130, 126, 125, 114, 104, 103, 95, 85, 80, 73, 64)
    SELECT "follows".* FROM "follows" WHERE "follows"."id" IN (14, 10)
    SELECT "favorites".* FROM "favorites" WHERE "favorites"."id" IN (25, 16, 14)
  </code></pre>
  
  <p>But when I display each ArtRating, I also need to reference the post title, which belongs to a post.  In the view, if I do:</p>
  
  <pre><code>activity.link.post
  </code></pre>
  
  <p>It does a separate DB call for each art_rating's post.  Is there a way to eagerly load the post as well?</p>
  
  <p><strong>UPDATE TO THE QUESTION:</strong></p>
  
  <p>If there is no way to achieve eager loading of posts using 'includes' syntax, is there a way to manually do the eager loading query myself and inject it into the @activities object?</p>
  
  <p>I see in the DB log: </p>
  
  <pre><code>SELECT "art_ratings".* FROM "art_ratings" WHERE "art_ratings"."id" IN (137, 136, 133, 130, 126, 125, 114, 104, 103, 95, 85, 80, 73, 64)
  </code></pre>
  
  <p>Is there a way I can access this list of ids from the @activities object?  If so, I could do 2 additional queries, 1 to get the art_ratings.post_id(s) in that list, and another to SELECT all posts IN those list of post_ids.  Then somehow inject the 'post' results back into @activities so that it's available as activity.link.post when I iterate through the collection.  Possible?</p>