Mongoid::VERSION # => "7.0.1"

class A
  include Mongoid::Document
  embeds_one :b
end

class B
  include Mongoid::Document
  embedded_in :a
  field :memo, type: String
end

a = A.create(b: { memo: 'before' })
a.b.memo # => "before"

a.b = nil
a.b # => nil

a.save
a.reload

# I expect it to be nil
a.b # => #<B _id: 5b33301f13909416dbc0d480, memo: "before">