When setting an _ids field of a has_and_belongs_to_many to a new array with the same elements as before but a different order, and then calling save, it's not persisted. When reading it later, the order is still as it was before.

Example:

class A
  include Mongoid::Document
  has_and_belongs_to_many :bs
end

class B
  include Mongoid::Document
end

And then:

b1 = B.create
b2 = B.create
a = A.create
a.b_ids = [b1.id, b2.id]
a.save

a.b_ids = [b2.id, b1.id]
a.save

a_reread = A.find(a.id)
a_reread.b_ids  #=> is [b1.id, b2.id], but should be [b2.id, b1.id]

When changing it directly via MongoDB shell, the order is correct (and Mongoid returns them in the new order).

My use case is that the user ranks a bunch of entries by ordering them, which should be persisted as an array of ids in mongo. So, preserving order is kind of important in this case.

Spec coming up.