Question

Im using mongomapper to back a rails app with mongodb and need to query across associations for a given dataset.

for eg.

class User
  include Mongomapper::Document

  key :age, Integer
  many :talents
end 

class Talent
  include Mongomapper::Document

  key :name, String
end

Now if I want to search for a user who is age 31, I can easily do

User.find_by_age(31).

but how do I search for a user with age 31 who has Juggling as a talent ??
Something like this:

User.find_by_age_and_talent(31, "juggling")

Doesnt have to be through mongomapper…could be a direct mongodb query.

Answer 1

Note that you have typos in your models, Mongomapper should be MongoMapper, catch this to save yourself some puzzlement.

Your models specify separate documents, and thus your search criteria are in separate documents. So you cannot specify a single query that spans documents, that’s the equivalent of a join, and there’s no join in MongoDB. So as written, you are stuck with exhaustive secondary querying of a primary results set.

However, if you modify Talent to be an embedded document, then you can do a single query in MongoDB, thus taking advantage of the beauty of MongoDB for both storage and querying, without the join overhead required by SQL.

To run either version, just change the include in the following model to be either MongoMapper::Document or MongoMapper::EmbeddedDocument. The test file supports both without modification.

class Talent
  #include MongoMapper::Document
  include MongoMapper::EmbeddedDocument

  key :name, String
end

test/unit/user_test.rb

require 'test_helper'

class UserTest < ActiveSupport::TestCase
  def setup
    User.delete_all
    Talent.delete_all if Talent.respond_to?(:delete_all)
  end

  test "user talent" do
    if Talent.respond_to?(:create)
      puts "Talent as separate document, not embedded"
      User.create(age: 31, talents: [Talent.create(name: 'juggling')])
      User.create(age: 31, talents: [Talent.create(name: 'singing')])
      User.create(age: 25, talents: [Talent.create(name: 'juggling'), Talent.create(name: 'dancing')])
      assert_equal(3, User.count)
      talent = 'juggling'
      users = User.where(age: 31).select{|user| Talent.where(name: talent).where(user_id: user._id).first}
      assert_equal(1, users.size)
      p users
    else
      puts "Talent as embedded document"
      User.create(age: 31, talents: [Talent.new(name: 'juggling')])
      User.create(age: 31, talents: [Talent.new(name: 'singing')])
      User.create(age: 25, talents: [Talent.new(name: 'juggling'), Talent.new(name: 'dancing')])
      assert_equal(3, User.count)
      talent = 'juggling'
      users = User.where(age: 31).where('talents.name' => talent).to_a
      assert_equal(1, users.size)
      p users
    end
  end
end