Rails：一次为 has_many 和 has_many 获取记录：通过关联

Question

Given the following models:

User

class User < ActiveRecord::Base
  has_many :given_loans,    :class_name => "Loan", :foreign_key => "lender_id"
  has_many :received_loans, :class_name => "Loan", :foreign_key => "borrower_id"
  has_many :borrowed_books, :class_name => "Book", :foreign_key => "borrower_id", :through => :received_loans
  has_many :own_books,      :class_name => "Book", :foreign_key => "owner_id"
end

Book

class Book < ActiveRecord::Base
  belongs_to :owner,     :class_name => "User", :foreign_key => "owner_id"
  has_many   :loans,     :foreign_key => "borrowed_book_id"
  has_many   :borrowers, :through => "loans",   :foreign_key => "borrowed_book_id"
end

Loan

class Loan < ActiveRecord::Base
  belongs_to :borrowed_book, :class_name => "Book", :foreign_key => "borrowed_book_id"
  belongs_to :borrower,      :class_name => "User", :foreign_key => "borrower_id"
  belongs_to :lender,        :class_name => "User", :foreign_key => "lender_id"
end

These relationships seem to work fine.

Now I'd like to query all books for a user, including both the borrowed books and the owned books, which I'm currently doing like this:

def books
  own_books + borrowed_books
end

This method naturally causes two SQL queries, though:

Book Load (0.4ms)  SELECT "books".* FROM "books" WHERE ("books".owner_id = 1)
Book Load (0.3ms)  SELECT "books".* FROM "books" INNER JOIN "loans" ON "books".id = "loans".borrowed_book_id WHERE (("loans".borrower_id = 1))

Furthermore I'm not able to run active record methods such as limit or order against this selection, which would be nice to have. After all I'm fetching the books from the same table.

I'm assuming that there's a better (more "Rails'") and more efficient way of doing this. Anyone care to point me into the right direction ? Cheers!

---

PS: A possible solution that came to mind was to define a query for a has_many :books relationship, but that didn't seem quite right either.

Answer 1

Here's what I ended up doing.

Using the meta_where gem the following method is possible to improve the situation:

def books
  Book.includes(:loans).where({ "owner_id" => self.id } | { "loans.borrower_id" => self.id })
end

This method returns a relation object and produces only one SQL query. Any opinions where this method should be located? Right now I'm having it in the User class.

Answer 2

Use this in the model:

def as_json(options={})
super(include: { trades: { include:  :trades } })
end