如果涉及多态关联，如何创建“ has_many through”关联？

Question

I have 4 models: Model 1 associated with model 2, and model 2 with model 3. Models 2 and 3 are associated to model 4 with a polymorphic association. This all works. I'm having trouble specifying the association between model 1 and model 4, which are thus related in two ways:

1. model 1 - model 2 - model 4
2. model 1 - model 2 - model 3 - model 4

Below you'll find all the associations, except that between model 1 and model 4:

---

Author model:

has_many :books,     dependent: :destroy

Book model:

belongs_to :author
has_many   :chapters, dependent: :destroy
has_many   :titles,   as: :titlesource,       # Polymorphic Association
                      dependent: :destroy

Chapter model:

belongs_to :book
has_many   :titles,   as: :titlesource,       # Polymorphic Association
                      dependent: :destroy

Title model:

belongs_to :titlesource, polymorphic: true

---

Question: Suppose you have the above associations and you want to know the titles that an author has, irrespective whether they are a book-title or chapter-title. What associations should I expand the above setup with? (I assume something with has_many through )

Update: If it were only book-titles I would be looking for, I would expect adding has_many :titles, through: :books to the Author model to work. But it doesn't. If I add that association, then Author.first.titles produces the error undefined method 'titles' . I think this has to do with the polymorphic association and naming...? What am I doing wrong?

For chapter title it is even more difficult. That association goes through the Book model, then through the Chapter model and then to the Title model. How can I include these titles in the author model? I think it would have to be code with the following logic (but obviously this code is incorrect):

has_many :titles, through: :books && :books:chapters

Update: The two answers given use SQL. Do these solutions not send a lot of queries to the db, causing problems if the method is used often? Has one of these methods an advantage over the other in performance?

Answer 1

If you were to use an association on the Author like has_many :titles, through: :books it should normally work as long as you have set a titlesource_id and a titlesource_type on the Title model migration.

However, if you would add another one to fetch the titles through the chapters like this has_many :titles, through: :chapters it would overwrite the first one and it would fetch only the titles that are of titlesource_type = 'Chapter' .

Therefore the solution that I would suggest to you, if you don't mind sql is to implement a method like this in the Author model.

  def titles
    Title.where(
      "(titles.titlesource_type = 'Book'
      AND titles.titlesource_id
      IN (SELECT books.id FROM books WHERE books.author_id = #{id}))
      OR (titles.titlesource_type = 'Chapter'
      AND titles.titlesource_id
      IN (SELECT chapters.id
      FROM books
      INNER JOIN chapters ON chapters.book_id = books.id
      WHERE books.author_id = #{id}))"
    )
  end

Answer 2

If you don't need ActiveRecord methods (like where or order ), then this will give you an array of titles.

def titles_array  # sacrificing ActiveRecord methods
  books.collect { |b| b.titles + b.chapters.collect { |c| c.titles } }
end

You can still use AR methods on the individual array elements such as titles_array.first.author , etc