Rails: includes with polymorphic association

Question

I read this interesting article about Using Polymorphism to Make a Better Activity Feed in Rails.

We end up with something like

class Activity < ActiveRecord::Base   belongs_to :subject, polymorphic: true end 

Now, if two of those subjects are for example:

class Event < ActiveRecord::Base   has_many :guests   after_create :create_activities   has_one :activity, as: :subject, dependent: :destroy  end  class Image < ActiveRecord::Base   has_many :tags   after_create :create_activities   has_one :activity, as: :subject, dependent: :destroy  end 

With create_activities defined as

def create_activities   Activity.create(subject: self) end 

And with guests and tags defined as:

class Guest < ActiveRecord::Base   belongs_to :event end  class Tag < ActiveRecord::Base   belongs_to :image end 

If we query the last 20 activities logged, we can do:

Activity.order(created_at: :desc).limit(20) 

We have a first N+1 query issue that we can solve with:

Activity.includes(:subject).order(created_at: :desc).limit(20) 

But then, when we call guests or tags, we have another N+1 query problem.

What's the proper way to solve that in order to be able to use pagination ?

Answer 1

Edit 2: I'm now using rails 4.2 and eager loading polymorphism is now a feature :)

Edit: This seemed to work in the console, but for some reason, my suggestion of use with the partials below still generates N+1 Query Stack warnings with the bullet gem. I need to investigate...

Ok, I found the solution ([edit] or did I ?), but it assumes that you know all subjects types.

class Activity < ActiveRecord::Base   belongs_to :subject, polymorphic: true    belongs_to :event, -> { includes(:activities).where(activities: { subject_type: 'Event' }) }, foreign_key: :subject_id   belongs_to :image, -> { includes(:activities).where(activities: { subject_type: 'Image' }) }, foreign_key: :subject_id end 

And now you can do

Activity.includes(:part, event: :guests, image: :tags).order(created_at: :desc).limit(10) 

But for eager loading to work, you must use for example

activity.event.guests.first 

and not

activity.part.guests.first 

So you can probably define a method to use instead of subject

def eager_loaded_subject   public_send(subject.class.to_s.underscore) end 

So now you can have a view with

render partial: :subject, collection: activity 

A partial with

# _activity.html.erb render :partial => 'activities/' + activity.subject_type.underscore, object: activity.eager_loaded_subject 

And two (dummy) partials

# _event.html.erb <p><%= event.guests.map(&:name).join(', ') %></p>  # _image.html.erb <p><%= image.tags.first.map(&:name).join(', ') %></p> 

Answer 2

This will hopefully be fixed in rails 5.0. There is already an issue and a pull request for it.

https://github.com/rails/rails/pull/17479

https://github.com/rails/rails/issues/8005

I have forked rails and applied the patch to 4.2-stable and it works for me. Feel free to use my fork, even though I cannot guarantee to sync with upstream on a regular basis.

https://github.com/ttosch/rails/tree/4-2-stable