class Ho
  include Mongoid::Document

  embeds_many :heys
  field :name, type: String
  validates_presence_of :heys
end

class Hey
  include Mongoid::Document

  embedded_in :ho
  field :name, type: String
end

ho = Ho.new
ho.heys = [Hey.new]
ho.save!
ho.heys
>> [#<Hey _id: 5295387669a951f9a4000004, name: nil>]

ho.update_attributes!({"heys"=>nil})
>> MOPED: 127.0.0.1:27017 UPDATE  database=database_name collection=hos selector={"_id"=>"529537f669a951f9a4000002"} update={"$pullAll"=>{"heys"=>[{"_id"=>"5295387669a951f9a4000004"}]}} flags=[] (0.2277ms)
>> Mongoid::Errors::Validations: 
Problem:
  Validation of Ho failed.
Summary:
  The following errors were found: Heys translation missing: pt-BR.mongoid.errors.models.ho.attributes.heys.blank
Resolution:

ho.reload
ho.heys
>> []