class Foo
  include Mongoid::Document
  has_many :bars
end

class Bar
  include Mongoid::Document
  belongs_to :foo
end

bars = 2.times.map { Bar.create! }
foo = Foo.create! :bars => bars
foo.bars = []
foo.bars(true)
 => [#<Bar _id: 5097894b0bed3f764600000f, _type: nil, foo_id: "5097894f0bed3f7646000010">]
Bar.all.map(&:foo_id)
 => [nil, "5097894f0bed3f7646000010"]