class Coin
include Mongoid::Document
has_and_belongs_to_many :boxes
validates :boxes, presence: true
end

class Box
include Mongoid::Document
has_and_belongs_to_many :coins
end

>> 2.times

{ Coin.create }

>> Box.create

{ coins: [Coin.first,Coin.last] }

When I type Coin.first.boxes and Coin.last.boxes it returns "nil" in both cases instead of the box i have just created.

This behaviour disappears by removing the validation in Coin, then it behaves as expected.

(Using ruby 2.0)