can't re-add live objects back into a relation

gistfile1.rb

class A
  include Mongoid::Document
  has_many :bs
end

class B
  include Mongoid::Document
  belongs_to :a
end

# populate
a = A.create
the_bs = [B.create, B.create]
a.bs << the_bs
a.save

# what do we have?
a.bs.count
# => 2

# now reset the relation
a.bs.nullify

# and now?
a.bs
# => []
a.bs.count
# => 0

# ok, let's add _something_
a.bs << the_bs[0]
# => [#<B _id: 5078fc1e5f2869aa4e000007, _type: nil, a_id: "5078fc1d5f2869aa4e000006">]

# Right! But...
a.bs.count
# => 0

# uh-oh....
a.bs.map(&:a_id)
# => ["5078fc1d5f2869aa4e000006"]
a.bs.map(&:persisted?) 
=> [true]

## at this point, the "B" object has its :a_id set to nil in the database, but mongoid
#  believes it's already persisted and refuses to write it, ever. the array and the state
#  in the database have diverged, and the database will never pick it up.
a.bs.each(&:save!)
# => true
a.bs.count
# => 0

# the same can be achieved by assigning "A" directly
the_bs[1].a
# => nil
the_bs[1].a = a
# => #<A _id: 5078fc1d5f2869aa4e000006, _type: nil>
the_bs[1].persisted?
# => true
the_bs[1].save!
# => true
the_bs[1].a
# => #<A _id: 5078fc1d5f2869aa4e000006, _type: nil>
the_bs[1].reload
the_bs[1].a
# => nil