Hi,

Not sure if I'm misusing nested attributes. Here's what I'm trying to do:

class Favorite
include Mongoid::Document

belongs_to :person
end

class Person
include Mongoid::Document

has_many :favorites

accepts_nested_attributes_for :favorites
end

> favorite = Favorite.create
> person = Person.create
> person.update_attributes(favorites_attributes: [favorite.attributes])
NoMethodError (undefined method `update_attributes' for nil:NilClass)
from mongoid-3.0.9/lib/mongoid/relations/builders/nested_attributes/many.rb:113:in `process_attributes'
from mongoid-3.0.9/lib/mongoid/relations/builders/nested_attributes/many.rb:29:in `block in build'
from mongoid-3.0.9/lib/mongoid/relations/builders/nested_attributes/many.rb:27:in `each'
from mongoid-3.0.9/lib/mongoid/relations/builders/nested_attributes/many.rb:27:in `build'
from mongoid-3.0.9/lib/mongoid/nested_attributes.rb:55:in `block (3 levels) in accepts_nested_attributes_for'
from mongoid-3.0.9/lib/mongoid/threaded/lifecycle.rb:26:in `_assigning'
from mongoid-3.0.9/lib/mongoid/nested_attributes.rb:54:in `block (2 levels) in accepts_nested_attributes_for'
from mongoid-3.0.9/lib/mongoid/attributes/processing.rb:137:in `block in process_nested'
from mongoid-3.0.9/lib/mongoid/attributes/processing.rb:136:in `each_pair'
from mongoid-3.0.9/lib/mongoid/attributes/processing.rb:136:in `process_nested'
from mongoid-3.0.9/lib/mongoid/attributes/processing.rb:150:in `process_pending'
from mongoid-3.0.9/lib/mongoid/attributes/processing.rb:32:in `block in process_attributes'
from mongoid-3.0.9/lib/mongoid/attributes/processing.rb:190:in `with_mass_assignment'
from mongoid-3.0.9/lib/mongoid/attributes/processing.rb:22:in `process_attributes'
from mongoid-3.0.9/lib/mongoid/attributes.rb:158:in `block in assign_attributes'
from mongoid-3.0.9/lib/mongoid/threaded/lifecycle.rb:26:in `_assigning'
from mongoid-3.0.9/lib/mongoid/attributes.rb:157:in `assign_attributes'
from mongoid-3.0.9/lib/mongoid/persistence.rb:175:in `update_attributes'

Mongoid blows up because it doesn't find a Favorite with the id contained in the nested attribute passed to update_attributes related to the person object.

What do you think about mongoid:
1. recognizing that the person object is not related to a Favorite with an id equal to the one contained in the nested attribute passed to update_attributes
2. inserting the favorite's id in the person object's favorite_ids field

I'm using mongoid 3.0.9, rails 3.2.8 and ruby 1.9.3p194.