class Order
  include Mongoid::Document
end

class User
  include Mongoid::Document
  embeds_many :addresses
end

class Address
  include Mongoid::Document
  embedded_in :user
  has_and_belongs_to_many :orders, :inverse_of => nil
end

Order.delete_all
User.delete_all

Order.create
=> #<Order _id: 4e6550dd2b58af32f2000007, _type: nil>

User.create :addresses => [{:orders => Order.first}]
Mongoid::Errors::InvalidCollection: Access to the collection for Address is not allowed since it is an embedded document, please access a collection from the root document.
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/collections.rb:55:in `collection'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/collections.rb:26:in `collection'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence/atomic/operation.rb:20:in `collection'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence/atomic/set.rb:22:in `block (2 levels) in persist'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence/atomic/set.rb:21:in `tap'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence/atomic/set.rb:21:in `block in persist'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence/atomic/operation.rb:78:in `prepare'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence/atomic/set.rb:19:in `persist'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence/atomic.rb:204:in `set'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/relations/referenced/many_to_many.rb:146:in `nullify'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/relations/referenced/many.rb:275:in `block in substitute'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/relations/referenced/many.rb:273:in `tap'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/relations/referenced/many.rb:273:in `substitute'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/relations/accessors.rb:126:in `block (2 levels) in setter'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/attributes/processing.rb:138:in `block in process_relations'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/attributes/processing.rb:133:in `each_pair'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/attributes/processing.rb:133:in `process_relations'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/attributes/processing.rb:121:in `process_pending'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/attributes/processing.rb:28:in `process'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/document.rb:130:in `block in initialize'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/relations/builders.rb:47:in `building'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/document.rb:127:in `initialize'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence.rb:170:in `new'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence.rb:170:in `block in create'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence.rb:273:in `creating'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/mongoid-2.2.0/lib/mongoid/persistence.rb:169:in `create'
        from (irb):22
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands/console.rb:44:in `start'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands/console.rb:8:in `start'
        from /usr/local/rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands.rb:23:in `<top (required)>'
        from script/rails:6:in `require'
        from script/rails:6:in `<main>'