两个belong_to引用同一张表+急切加载

Question

首先，基于这个（Rails association with multiple foreign keys）我想出了如何让两个belong_to指向同一张表。

我有类似的东西

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

一切都好。我可以做任何一个

• book.author
• book.co_author
• 作者.books

但是，有时我需要为多个作者急切加载书籍（以避免 N 次查询）。

我正在尝试做类似的事情：

Author.includes(books: :title).where(name: ["Lewis Carroll", "George Orwell"])

Rails 5 向我抛出：“ArgumentError: 关联范围 'books' 依赖于实例（范围块接受参数）。不支持预加载依赖于实例的范围。”

我想知道我应该怎么做？

我应该使用多对多关联吗？这听起来像是一个解决方案。但是，它看起来会引入它自己的问题（我需要“排序”，这意味着我需要明确区分主要作者和共同作者）。

只是想弄清楚我是否缺少一些更简单的解决方案......

Answer 1

试试这个：

 Author.where(name: ["Lewis Carroll", "George Orwell"]).include(:books).select(:title)

Answer 2

为什么不使用HABTM 关系？例如：

# 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

你可以像下面这样使用 eagerload：

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]]