ActiveRecord has_many through polymorphic has_many - sql

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