Many to many with polymorphic association doesn't work in both directions

Question

I am implementig the system that enable user to follow the "followable"( in my case these may be an event, place or other user).

My idea:

Follow model holds the user_id, followable type and followale_id(join table)

class Follow < ActiveRecord::Base
  belongs_to :user
  belongs_to :followable, polymorphic: true
end

Event

class Event < ActiveRecord::Base    
  has_many :follows, as: :followable
  has_many :users, through: :follows
end

Place

class Place < ActiveRecord::Base
  has_many :follows, as: :followable
  has_many :users, through: :follows    
end

User

class User < ActiveRecord::Base
  has_many :follows
  has_many :events, through: :follows, source: :followable, source_type: "Event"
  has_many :places, through: :follows, source: :followable, source_type: "Place"
  has_many :users, through: :follows, source: :followable, source_type: "User"
end

The problem is that the realtion works only in one direction, i can do:

user.follows.create(followable:event1) #follow event1
user.follows.create(followable:place1) #follow place1
user.follows.create(followable:user1)  #follow user1
user.follows # displays all following relations user has established

But, i cannot do:

event1.follows #return follow objects(some user - event1 pairs)
event1.users #return all of the users that follow this event
user1.users  #return all of the users that user1 follows, the most confusing part..

All of the aboves return nil.

How should i establish the relations to make it work in both directions?
Also, i'd like to hear some remarks on how to improve this idea, beacouse it's the first time i'm playin around with more complex realtions. Thank you in advance.

Answer 1

Lets start off with the User model as its the trickiest:

class User < ActiveRecord::Base
  has_many :follows, source: :user
  has_many :follows_as_fallowable, 
                    class_name: 'Follow',
                    as: :followable

  has_many :followers, through: :follows_as_fallowable,
                       source: :user

  # other users the user is following                     
  has_many :followed_users, through: :follows,
                            source: :followable,
                            source_type: 'User'
end

Note that we need two different relationships to follows due to the fact that the user can be in either column depending on if the user is the follower or the object being followed.

We can now do a simple test to check if the relationship is setup properly:

joe = User.create(name: 'Joe')
jill = User.create(name: 'Jill')
joe.followers << jill
jill.followed_users.include?(joe) # true
joe.followers.include?(jill) # true

To then setup a the bi-directional relation between users and a followable model you would do:

class Event < ActiveRecord::Base
  has_many :follows, as: :followable
  has_many :followers, through: :follows,
                       source: :user
end

class User < ActiveRecord::Base
  # ...
  has_many :followed_events, through: :follows,
                             source: :followable,
                             source_type: 'Event'
end

The relations in the followed model (Event) are pretty much the same in every model so you can easily extract it to a module for reuse:

# app/models/concerns/followable.rb
module Followable
  extend ActiveSupport::Concern

  included do
    has_many :follows, as: :followable
    has_many :followers, through: :follows,
                         source: :user
  end
end

class Event < ActiceRecord::Base
  include Followable
end

class Place < ActiceRecord::Base
  include Followable
end