is there a way to eager load polymorphic association's associations?

Question

artists have many activities (basically a cache of interactions between users):

class Activity < ActiveRecord::Base

  belongs_to :receiver, :class_name => 'Artist', :foreign_key => :receiver_id #owns the stuff done "TO" him
  belongs_to :link, :polymorphic => true
  belongs_to :creator, :class_name => 'Artist', :foreign_key => :creator_id #person who initiated the activity

end

For example:

  Activity.create(:receiver_id => author_id, :creator_id => artist_id, :link_id => id, :link_type => 'ArtRating')

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.

The controller looks like this:

class ActivityStreamController < ApplicationController
  def index
    @activities = @artist.activities.includes([:link,:creator,:receiver]).order("id DESC").limit(30)
  end
end

The db call correctly eagerly loads the polymorphic link objects:

  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)

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:

activity.link.post

It does a separate DB call for each art_rating's post. Is there a way to eagerly load the post as well?

UPDATE TO THE QUESTION:

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?

I see in the DB log:

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)

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?

From what I understand, you have a set of models that are supposed to be attached to activities as links and I gather they all share at least one association, which is `post`. Is there maybe enough common ground in these link models to use [Single Table Inheritance](http://apidock.com/rails/ActiveRecord/Base) and put all in one table? — CMW, Oct 25 '13 at 13:03

Kaleidoscope · Answer 1 · 2013-11-01T01:20:20.863

TL;DR my solution makes artist.created_activities.includes(:link) eager load everything you want

Here's my ~~first~~ attempt at it: https://github.com/JohnAmican/music

A few notes:

~~I'm relying on default_scope, so this isn't optimal.~~
It looks like you're using STI. My solution doesn't. That means you can't simply call activities on an artist; you have to reference created_activities or received_activities. There might be a way around this. I'll update if I find anything.
I changed some names around because it was confusing to me otherwise.

~~If you go into console and do created_activities.includes(:link), the appropriate stuff gets eager-loaded:~~

irb(main):018:0> artist.created_activities.includes(:link)
  Activity Load (0.2ms)  SELECT "activities".* FROM "activities" WHERE "activities"."creator_id" = ?  [["creator_id", 1]]
  Rating Load (0.3ms)  SELECT "ratings".* FROM "ratings" WHERE "ratings"."id" IN (1)
  RatingExplanation Load (0.3ms)  SELECT "rating_explanations".* FROM "rating_explanations" WHERE "rating_explanations"."rating_id" IN (1)
  Following Load (0.3ms)  SELECT "followings".* FROM "followings" WHERE "followings"."id" IN (1)
  Favorite Load (0.2ms)  SELECT "favorites".* FROM "favorites" WHERE "favorites"."id" IN (1)
=> #<ActiveRecord::Relation [#<Activity id: 1, receiver_id: 2, creator_id: 1, link_id: 1, link_type: "Rating", created_at: "2013-10-31 02:36:27", updated_at: "2013-10-31 02:36:27">, #<Activity id: 2, receiver_id: 2, creator_id: 1, link_id: 1, link_type: "Following", created_at: "2013-10-31 02:36:41", updated_at: "2013-10-31 02:36:41">, #<Activity id: 3, receiver_id: 2, creator_id: 1, link_id: 1, link_type: "Favorite", created_at: "2013-10-31 02:37:04", updated_at: "2013-10-31 02:37:04">]>

~~At the very least, this proves that Rails has the ability to do this. Circumventing default_scope seems like an issue with telling Rails what you want to do rather than a technical limitation.~~

UPDATE:

Turns out that when you pass a scope block to an association and call that association, self within that block refers to the relation. So, you can reflect on it and act appropriately:

class Activity < ActiveRecord::Base
  belongs_to :creator,  class_name: 'Artist', foreign_key: :creator_id,  inverse_of: :created_activities
  belongs_to :receiver, class_name: 'Artist', foreign_key: :receiver_id, inverse_of: :received_activities
  belongs_to :link, -> { self.reflections[:activity].active_record == Rating ? includes(:rating_explanation) : scoped }, polymorphic: true
end

I updated my code to reflect (haha) this.

This can be cleaned up. For example, maybe you don't always want to eager load rating_explanations when accessing activity links. There are a number of ways to solve that. I could post one if you'd like.

But, I think the most important thing that this shows is that within the association's scope block, you have access to the ActiveRecord::Relation being built. This will allow you to do things conditionally to it.

score 0 · Answer 2 · answered Oct 25 '13 at 14:01

If I get this right, you not only want to load a polymorphic association's association, but a polymorphic association's polymorphic association.

This basically means joining one defined table with another defined table through a bunch of undefined tables that come from some fields in the database.

Since both the activity and the post are joined through a polymorphic object they both have a something_id and something_type column.

Now I think active record doesn't let you do this out of the box but basically you want something like:

class Activity
  has_one :post, :primary_key => [:link_id, :link_type], :foreign_key => [:postable_id, :postable_type]
end

(assuming your polymorphic association on Post is belongs_to :postable)

That would then give you a query sort-of direct association between Post and Activity that is a very weird take on habtm with a 'polymorphic join table' (I just made that term up). Because both share the same polymorphicly associated object, they can be connected. Sort of like a friends-of-my-friends thing.

CAVEAT: Like I said, as far as I know, AR doesn't let you do this out of the box (though it would be awesome) but there are some gems that give you composite foreign and/or primary keys. Maybe you can find one to help you solve your problem in this way.

score 0 · Answer 3 · answered Oct 27 '13 at 18:54

Given your update to the question (that this isn't feasible using AR's include), you could try the following to get the associated Posts with a single extra query:

fetch the Activities, then build an Array of their linked post_ids.

@activities = @artist.activities.includes([:link,:creator,:receiver])
activity_post_ids = @activities.map{|a| a.link.post_id}.compact

then load the posts in one query and store them in a Hash, indexed by their id

activity_posts = Hash[Post.where(id: activity_post_ids).map{|p| [p.id, p]}]
#=> {1 => #<Post id:1>, 3 => #<Post id:3>, ... }

finally loop over @activities and set the post attribute of each associated link
```
@activities.each{|a| a.link.post = activity_posts[a.link.post_id]}
```

score 0 · Answer 4 · answered Nov 01 '13 at 01:42

This simple ActiveRecord should work:

@artist.activities.includes([:link => :post,:creator,:receiver]).order("id DESC").limit(30)

If not, if you are getting an error like "Association named 'post' was not found; perhaps you misspelled it?", then you have a model problem, because at least one of your link associations doesn't have the post association.

Polymorphic associations are intended to be used with a common interface. If you ask for the post for any of the polymorphic association, then ALL of your associations should implemented also that association (the post one).

Check that all of your models used in the polymorphic association implement the post association.

score 0 · Answer 5 · answered Dec 28 '11 at 07:00

0

Try this:

@artist.activities.includes([{:link => :post},:creator,:receiver])...

See the Rails docs for more.

answered Dec 28 '11 at 07:00

rkb

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normally that would work, but docs don't say anything about eager loading through the polymorphic association. Get error: "Association named 'post' was not found; perhaps you misspelled it?" – Homan Dec 28 '11 at 16:56
I tested this and it works fine (3.2.14). Are you sure all your "link" associations have the post association? – aromero Nov 01 '13 at 01:33

is there a way to eager load polymorphic association's associations?

5 Answers5