When using eager loading, only existing related documents are added to the identity map. This may sound like exactly what you'd want but in fact presents problems when lots of parent documents don't have any related documents. Consider the below:

class A
  include Mongoid::Document
  belongs_to :b
end

class B
  include Mongoid::Document
  has_one :a
end

5.times { B.create }
A.create(:b => B.last)

Mongoid.identity_map_enabled = true

B.all.includes(:a).collect(&:a)

In that example, 6 queries will be executed:

(1) Find all Bs – perfectly fine
(2) Find all As with matching b_ids – perfectly fine
(3)–(6) For all Bs with no matching A, try to find it again because the identity map does not store nil objects – potentially huge performance problem

Not sure if this is worth "fixing", or on what level that should be done (identity map or relations), but if it's a wontfix, I would suggest adding it as a potential caveat to the docs.