model embedded collection of object_ids with mongoid

gistfile1.txt

If you set has_and_belongs_to_many on one side only, then you need to 
set :inverse_of to nil. For example: 
class A 
  include Mongoid::Document 
  has_and_belongs_to_many :bs, :inverse_of => nil 
end 
class B 
  include Mongoid::Document 
end 
That should work with no trouble. Since you do not have the 
relationship set up on the B side, you must do everything via A's 
accessors. 
The has_and_belongs_to_many macro will try to set up relationship on 
both sides. Consider the following: 
a = A.new 
b = B.new 
a.bs << b 
After adding b to a's collection of B's, Mongoid will try to access 
b's collection of A's and add a to that collection. The name of the 
relationship on the other side is declared by :inverse_of. By default, 
Mongoid will expect b to have a collection named as. Since B does not 
have this relationship, we must tell Mongoid that there is no inverse, 
ie. :inverse_of => nil.