Rails Mongoid 搜索至少有一个子元素的父元素

Question

我有一个 rails 4 应用程序，我有 2 个对象：商店和书籍（引用 1:n 关系）。 一家商店有很多书，每本书都属于一家商店。有些商店没有书。 如何进行查询以找到至少有 1 本书的 3 家最新的商店 - 以及没有书籍的 3 家最新的商店？

@stores = Store.order_by(:created_at => 'desc').limit(4).uniq
@books = Book.order_by(:created_at => 'desc').limit(4).uniq

这项工作，但我不知道如何为商店或 @books 做一个 where(book.exists?) 以确保每本书都属于一个独特的商店。

Answer 1

MongoDB 中的基本 CRUD 操作对 单个 集合进行操作， “加入”功能尚未实现，这将允许它们跨两个集合进行操作。 一旦你理解了这一点，你就可以很容易地构造两个操作来做你想做的事情。 鉴于您提供的架构提示，以下是最明显的。 要执行更多操作，您可能需要考虑架构更改，例如嵌入。

根据定义，您提供的架构只允许一本书属于一个（根据定义是唯一的）商店。 您可以从对书籍的检查中看到这一点，每本书都有一个值 store_id。 希望这有助于您的理解。

app/models/store.rb

class Store
  include Mongoid::Document
  include Mongoid::Timestamps
  field :name, type: String

  has_many :books
end

app/models/book.rb

class Book
  include Mongoid::Document
  include Mongoid::Timestamps
  field :title, type: String

  belongs_to :store
end

test/unit/store_test.rb

需要'test_helper' 需要'pp'

class StoreTest < ActiveSupport::TestCase
  def setup
    Mongoid.default_session.drop
  end
  test '0. mongoid version' do
    puts "\nMongoid::VERSION:#{Mongoid::VERSION}\nMoped::VERSION:#{Moped::VERSION}"
  end
  test 'store query has book, does not have book' do
    [
        ["Amazon.com", ["Outlander", "Taking It All"]],
        ["Barnes & Noble", ["Big Little Lies"]],
        ["Goodreads", []],
        ["Greenlight Bookstore", []],
        ["Powell's Books", ["Gone Girl", "Dark Skye"]],
        ["Strand Books", []]
    ].each do |store, books|
     store = Store.create(name: store)
     books.each do |title|
       store.books << Book.create(title: title)
     end
     sleep 1
    end
    assert_equal(6, Store.count)
    assert_equal(5, Book.count)
    puts
    store_ids_with_books = Book.distinct(:store_id)
    latest_stores_with_a_book = Store.in(_id: store_ids_with_books).order_by(:created_at => 'desc').limit(3).to_a
    puts "three latest stores with a book:"
    pp latest_stores_with_a_book
    latest_stores_without_a_book = Store.nin(_id: store_ids_with_books).order_by(:created_at => 'desc').limit(3).to_a
    puts "three latest stores without a book:"
    pp latest_stores_without_a_book
    puts "books:"
    pp Book.all.to_a
  end
end

耙式测试

Run options: 

# Running tests:

[1/2] StoreTest#test_0._mongoid_version
Mongoid::VERSION:3.1.6
Moped::VERSION:1.5.2
[2/2] StoreTest#test_store_query_has_book,_does_not_have_book
three latest stores with a book:
[#<Store _id: 53f257287f11ba75e5000008, created_at: 2014-08-18 19:42:32 UTC, updated_at: 2014-08-18 19:42:32 UTC, name: "Powell's Books">,
 #<Store _id: 53f257257f11ba75e5000004, created_at: 2014-08-18 19:42:29 UTC, updated_at: 2014-08-18 19:42:29 UTC, name: "Barnes & Noble">,
 #<Store _id: 53f257247f11ba75e5000001, created_at: 2014-08-18 19:42:27 UTC, updated_at: 2014-08-18 19:42:27 UTC, name: "Amazon.com">]
three latest stores without a book:
[#<Store _id: 53f257297f11ba75e500000b, created_at: 2014-08-18 19:42:33 UTC, updated_at: 2014-08-18 19:42:33 UTC, name: "Strand Books">,
 #<Store _id: 53f257277f11ba75e5000007, created_at: 2014-08-18 19:42:31 UTC, updated_at: 2014-08-18 19:42:31 UTC, name: "Greenlight Bookstore">,
 #<Store _id: 53f257267f11ba75e5000006, created_at: 2014-08-18 19:42:30 UTC, updated_at: 2014-08-18 19:42:30 UTC, name: "Goodreads">]
books:
[#<Book _id: 53f257247f11ba75e5000002, created_at: 2014-08-18 19:42:28 UTC, updated_at: 2014-08-18 19:42:28 UTC, title: "Outlander", store_id: "53f257247f11ba75e5000001">,
 #<Book _id: 53f257247f11ba75e5000003, created_at: 2014-08-18 19:42:28 UTC, updated_at: 2014-08-18 19:42:28 UTC, title: "Taking It All", store_id: "53f257247f11ba75e5000001">,
 #<Book _id: 53f257257f11ba75e5000005, created_at: 2014-08-18 19:42:29 UTC, updated_at: 2014-08-18 19:42:29 UTC, title: "Big Little Lies", store_id: "53f257257f11ba75e5000004">,
 #<Book _id: 53f257287f11ba75e5000009, created_at: 2014-08-18 19:42:32 UTC, updated_at: 2014-08-18 19:42:32 UTC, title: "Gone Girl", store_id: "53f257287f11ba75e5000008">,
 #<Book _id: 53f257287f11ba75e500000a, created_at: 2014-08-18 19:42:32 UTC, updated_at: 2014-08-18 19:42:32 UTC, title: "Dark Skye", store_id: "53f257287f11ba75e5000008">]
Finished tests in 6.193234s, 0.3229 tests/s, 0.3229 assertions/s.             
2 tests, 2 assertions, 0 failures, 0 errors, 0 skips