BSON::InvalidDocument: Cannot serialize an object into BSON - mongodb

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