Rails Mongoid search parents elements that have at least one child - search

Question

I have a rails 4 app and I have 2 objects: stores and books (referenced 1:n relation).
A store has many books, each book belongs to one store. Some stores don't have any books.
How can I make a query to find the 3 latest store that has at least 1 book - and the 3 latest stores that have no books ?
#stores = Store.order_by(:created_at => 'desc').limit(4).uniq
#books = Book.order_by(:created_at => 'desc').limit(4).uniq
This work but I don't know how to do a where(book.exists?) for store, or for the #books to make sure each book belongs to a unique store.

Answer 1

Basic CRUD operations in MongoDB operate on a single collection,
"join" capability is not yet implemented that would allow them to operate across two collections.
Once you understand this, you can easily construct two operations that will do what you want.
The following is the most obvious given the schema hints that you supplied.
To do anything more, you probably want to consider schema changes like embedding.
By definition, your supplied schema only allows a book to belong to one (unique by definition) store.
You can see this from the inspection of the books, that there is a single value store_id for each book.
Hope that this helps your understanding.
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
require 'test_helper'
require '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
rake test
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