rails 4通过关联在has_many上使用includes时保留连接对象顺序

Question

Using Rails 4.1.6 and Ruby 2.1.2 and given the following three objects:

class FilmCollection
  has_many :film_collection_films, -> { order(:position) }
  has_many :films, through: :film_collection_films
end

class FilmCollectionFilm
  belongs_to :film_collection_film
  belongs_to :film
end

class Film
  has_many :film_collection_films
end

FilmCollections are collections of films, and its members are represented via the join object FilmCollectionFilm. FilmCollectionFilm has a column for position, so that a given member of a collection can be re-ordered (think like a film queue) and thus why we need the join object and not a has_and_belongs_to_many.

My problem occurs when I try to eager load the films of multiple collections at once.

Calling FilmCollection.first.films will get me a query that looks like:

SELECT `film_collections`.* FROM `film_collections`
  ORDER BY `film_collections`.`id` ASC LIMIT 1
SELECT `films`.* FROM `films`
  INNER JOIN `film_collection_films` ON `films`.`id` = `film_collection_films`.`film_id`
  WHERE `film_collection_films`.`extensional_film_collection_id` = 1
  ORDER BY `film_collection_films`.`position` ASC

Which orders the films correctly based on the position in the join object.

But instead calling FilmCollection.includes(:films).first.films will get me the following query:

SELECT `film_collections`.* FROM `film_collections`
  ORDER BY `film_collections`.`id` ASC LIMIT 1
SELECT `film_collection_films`.* FROM `film_collection_films`
  WHERE `film_collection_films`.`film_collection_id` IN (1)
  ORDER BY `film_collection_films`.`position` ASC
SELECT `films`.* FROM `films`  WHERE `films`.`id` IN (1, 16, 53, 185)

Which gathers the correct films but disregards the order in the second part of the query. How can I preserve ordering respective of the join object when eager loading with .includes() but still not have an n+1 query?

Answer 1

It turns out that you can solve this by explicitly including the join object as well as the association that is through it. The line below will select everything correctly, and preserve any conditionals/ordering present on the join object as well.

FilmCollection.includes(film_collection_films: :film)

If you do just FilmCollection.includes(:films) then it will select the FilmCollectionFilms anyway and then just toss them. So, the above code will do about the same amount of work but build the query correctly.