First of all, based on this ( Rails association with multiple foreign keys ) I figured out how to make two belong_to pointing to the same table.
I have something like that
class Book < ApplicationRecord
belongs_to :author, inverse_of: :books
belongs_to :co_author, inverse_of: :books, class_name: "Author"
end
class Author < ApplicationRecord
has_many :books, ->(author) {
unscope(:where).
where("books.author_id = :author_id OR books.co_author_id = :author_id", author_id: author.id)
}
end
It's all good. I can do either
However, sometimes I need to eager load books for multiple authors (to avoid N queries).
I am trying to do something like:
Author.includes(books: :title).where(name: ["Lewis Carroll", "George Orwell"])
Rails 5 throws at me: "ArgumentError: The association scope 'books' is instance dependent (the scope block takes an argument). Preloading instance dependent scopes is not supported."
I am trying to figure out what I should do?
Should I go with many-to-many association? It sounds like a solution. However, it looks like it will introduce it's own problems (I need "ordering", meaning that I need explicitly differentiate between main author and co-author).
Just trying to figure out whether I am missing some simpler solution...
Why do you not use HABTM relation? For example:
# Author model
class Author < ApplicationRecord
has_and_belongs_to_many :books, join_table: :books_authors
end
# Book model
class Book < ApplicationRecord
has_and_belongs_to_many :authors, join_table: :books_authors
end
# Create books_authors table
class CreateBooksAuthorsTable < ActiveRecord::Migration
def change
create_table :books_authors do |t|
t.references :book, index: true, foreign_key: true
t.references :author, index: true, foreign_key: true
end
end
end
You can use eagerload like as following:
irb(main):007:0> Author.includes(:books).where(name: ["Lewis Carroll", "George Orwell"])
Author Load (0.1ms) SELECT "authors".* FROM "authors" WHERE "authors"."name" IN (?, ?) LIMIT ? [["name", "Lewis Correll"], ["name", "George Orwell"], ["LIMIT", 11]]
HABTM_Books Load (0.1ms) SELECT "books_authors".* FROM "books_authors" WHERE "books_authors"."author_id" IN (?, ?) [["author_id", 1], ["author_id", 2]]
Book Load (0.1ms) SELECT "books".* FROM "books" WHERE "books"."id" IN (?, ?) [["id", 1], ["id", 2]]
尝试这个:
Author.where(name: ["Lewis Carroll", "George Orwell"]).include(:books).select(:title)
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