ActiveRecord has_many通过多态has_many

Question

It seems like rails still not support this type of relation and throws ActiveRecord::HasManyThroughAssociationPolymorphicThroughError error.

What can I do to implement this kind of relation?

I have following associations:

Users 1..n Articles
Categories n..n Articles
Projects 1..n Articles

And here is Subscription model

Subscription 1..1 User
Subscription 1..1 Target (polymorphic (Article, Category or User))

And I need to select articles through Subscription#target#article according to user#subscriptions.

I have no idea hot to implement this

Ideally I want to get instance of Association class

UPDATE 1

Here is little example

Let say user_1 has 4 Subscription records:

s1 = (user_id: 1, target_id: 3, target_type: 'User')
s2 = (user_id: 1, target_id: 2, target_type: 'Category')
s3 = (user_id: 1, target_id: 3, target_type: 'Project')
s4 = (user_id: 1, target_id: 8, target_type: 'Project')

I need method User#feed_articles, that fetches all articles, that belong to any of target, I subscribed.

user_1.feed_articles.order(created_at: :desc).limit(10) 

UPDATE 2

I separate articles sources by type in User model:

  has_many :out_subscriptions, class_name: 'Subscription'

  has_many :followes_users, through: :out_subscriptions, source: :target, source_type: 'User'
  has_many :followes_categories, through: :out_subscriptions, source: :target, source_type: 'Category'
  has_many :followes_projects, through: :out_subscriptions, source: :target, source_type: 'Project'

  has_many :feed_user_articles, class_name: 'Article', through: :followes_users, source: :articles
  has_many :feed_category_articles, class_name: 'Article', through: :followes_categories, source: :articles
  has_many :feed_project_articles, class_name: 'Article', through: :followes_projects, source: :articles

But how can I merge feed_user_articles with feed_category_articles and feed_project_articles without loss of perfomance

UPDATE 3.1

The only way I found is to use raw SQL join query. Looks like it works fine, but I'm not sure.

  def feed_articles
    join_clause = <<JOIN
inner join users on articles.user_id = users.id
inner join articles_categories on articles_categories.article_id = articles.id
inner join categories on categories.id = articles_categories.category_id
inner join subscriptions on
    (subscriptions.target_id = users.id and subscriptions.target_type = 'User') or
    (subscriptions.target_id = categories.id and subscriptions.target_type = 'Category')
JOIN

    Article.joins(join_clause).where('subscriptions.user_id' => id).distinct
  end

(This is just for Users and Categories)

It supports scopes and other features. The only thing interests me: does this query lead to some undesirable effect?

Answer 1

I think that from DB performance prospective using UNION ALL multiquery will be more efficient than using polymorphic multijoin. Also it will be more readable. I tried to write an Arel query as example but it does not play nice (I failed to make order by clause work properly) so I think you have to put it via raw SQL. You can use SQL template apart from ORDER BY clause for drying it up.

Answer 2

You are correct that Rails doesn't support has_many :through w/ polymorphic associations. You could mimic this behavior by defining an instance method on your User class. That would look something like this:

def articles
  Article.
    joins("join subscriptions on subscriptions.target_id = articles.id and subscriptions.target_type = 'Article'").
    joins("join users on users.id = subscriptions.user_id")
end