Question

I’m trying to follow along with http://mongotips.com/b/array-keys-allow-for-modeling-simplicity/

I have a Story document and a Rating document. The user will rate a story, so I wanted to create a many relationship to ratings by users as such:

class StoryRating
  include MongoMapper::Document

  # key <name>, <type>
  key :user_id, ObjectId
  key :rating, Integer
  timestamps!

end

class Story
  include MongoMapper::Document

  # key <name>, <type>
  timestamps!
  key :title, String
  key :ratings, Array, :index => true

  many :story_ratings, :in => :ratings

end

Then

irb(main):006:0> s = Story.create  
irb(main):008:0> s.ratings.push(Rating.new(user_id: '0923ksjdfkjas'))  
irb(main):009:0> s.ratings.last.save  
=> true  
irb(main):010:0> s.save  
BSON::InvalidDocument: Cannot serialize an object of class StoryRating into BSON.  
    from /usr/local/lib/ruby/gems/1.9.1/gems/bson-1.6.2/lib/bson/bson_c.rb:24:in `serialize' (...)

Why?

Answer 1

You should be using the association “story_rating” method for your push/append rather than the internal “rating” Array.push to get what you want to follow John Nunemaker’s “Array Keys Allow For Modeling Simplicity” discussion. The difference is that with the association method, MongoMapper will insert the BSON::ObjectId reference into the array, with the latter you are pushing a Ruby StoryRating object into the Array, and the underlying driver driver cant serialize it.

Here’s a test that works for me, that shows the difference. Hope that this helps.

Test

require 'test_helper'

class Object
  def to_pretty_json
    JSON.pretty_generate(JSON.parse(self.to_json))
  end
end

class StoryTest < ActiveSupport::TestCase
  def setup
    User.delete_all
    Story.delete_all
    StoryRating.delete_all
    @stories_coll = Mongo::Connection.new['free11513_mongomapper_bson_test']['stories']
  end

  test "Array Keys" do
    user = User.create(:name => 'Gary')
    story = Story.create(:title => 'A Tale of Two Cities')
    rating = StoryRating.create(:user_id => user.id, :rating => 5)
    assert_equal(1, StoryRating.count)
    story.ratings.push(rating)
    p story.ratings
    assert_raise(BSON::InvalidDocument) { story.save }
    story.ratings.pop
    story.story_ratings.push(rating) # note story.story_ratings, NOT story.ratings
    p story.ratings
    assert_nothing_raised(BSON::InvalidDocument) { story.save }
    assert_equal(1, Story.count)
    puts Story.all(:ratings => rating.id).to_pretty_json
  end
end

Result

Run options: --name=test_Array_Keys

# Running tests:

[#<StoryRating _id: BSON::ObjectId('4fa98c25e4d30b9765000003'), created_at: Tue, 08 May 2012 21:12:05 UTC +00:00, rating: 5, updated_at: Tue, 08 May 2012 21:12:05 UTC +00:00, user_id: BSON::ObjectId('4fa98c25e4d30b9765000001')>]
[BSON::ObjectId('4fa98c25e4d30b9765000003')]
[
  {
    "created_at": "2012-05-08T21:12:05Z",
    "id": "4fa98c25e4d30b9765000002",
    "ratings": [
      "4fa98c25e4d30b9765000003"
    ],
    "title": "A Tale of Two Cities",
    "updated_at": "2012-05-08T21:12:05Z"
  }
]
.

Finished tests in 0.023377s, 42.7771 tests/s, 171.1084 assertions/s.

1 tests, 4 assertions, 0 failures, 0 errors, 0 skips