class Foo
  include Mongoid::Document
  embeds_many :bars
end

class Bar
  include Mongoid::Document
  embedded_in :foo
  field :flag, type: Boolean
  default_scope where(flag: true)
end

f.bars.create!(flag: true)
f.bars.create!(flag: false)
f.bars
# => [#<Bar _id: 4e70a5f85c070f025c000003, _type: nil, flag: false>, #<Bar _id: 4e70a5fb5c070f025c000004, _type: nil, flag: true>]

f.bars.all.selector
# => {:flag=>true}   // ok
f.bars.unscoped.selector
# => {:flag=>true}   // wrong! should be {}