"Chained" relations (1:n:m) with Mongoid and Rails

Question

I'm trying to achieve a really simple "multiple relationship" in Rails (3.2.12) with Mongoid (3.1.2). I got three models:

class User
  include Mongoid::Document
  field :name
  has_many :collections
end

class Collection
  include Mongoid::Document
  field :name
  belongs_to :user
  has_many :things
end

class Thing
  include Mongoid::Document
  field :name
  belongs_to :collection
end

So basically there is a user who has collections and there are collections which have "Things". Well ... first question: Is this even possible? And if not: Why not? ;-)

I can create users now (from a rails console or via web), and I can create collections belonging to users. But when I try to create a "Thing" now ... well it just doesn't get created:

2.0.0-p0 :014 >   u = User.first
 => #<User _id: 514b2b32e05658e1f1000005, name: "test"> 
2.0.0-p0 :015 > c = Collection.create!(name: "somename", user: u)
 => #<Collection _id: 514b30f0e05658ca67000001, name: "somename", user_id: "514b2b32e05658e1f1000005"> 
2.0.0-p0 :016 > t = Thing.create!(name: "a thing", collection: c)
 => #<Thing _id: 514b3120e05658ca67000002, name: "a thing", collection_id: "514b30f0e05658ca67000001"> 
2.0.0-p0 :017 > Thing.first
 => nil 

The "Collection" gets created just fine:

2.0.0-p0 :018 > Collection.first
 => #<Collection _id: 514b2b65e05658e1f1000006, name: "test", user_id: "514b2b32e05658e1f1000005"> 

Answer 1

Change the Collection class name to something else, say to 'Stockpile'. MongoDB uses 'collection' as part of it's terminology, so it may not play well with your models.