# app/models/address.rb
class Address
  include Mongoid::Document

  field :city, type: String

  embedded_in :addressable, polymorphic: true
end

# app/models/company.rb
class Company
  include Mongoid::Document

  embeds_one :address, class_name: 'Address', as: :addressable
  accepts_nested_attributes_for :address

  embeds_one :delivery_address, class_name: 'Address', as: :addressable
  accepts_nested_attributes_for :delivery_address
end

2.5.3 :001 > acme = Company.create(address: { city: 'Gotham' }, delivery_address: { city: 'Parcelville' })
 => #<Company _id: 5cd32489a8f843353860cc0b, > 
2.5.3 :002 > acme.reload.address.city
 => "Gotham" 
2.5.3 :003 > acme.reload.delivery_address.city
 => "Parcelville" 
2.5.3 :004 > acme.update(delivery_address: { city: 'Smallville' })
 => true 
2.5.3 :005 > acme.reload.address.city
 => "Gotham" 
2.5.3 :006 > acme.reload.delivery_address.city
 => "Smallville"

2.5.3 :007 > acme.update(delivery_address: Address.new(city: 'Bigville'))
 => true 
2.5.3 :008 > acme.reload.address.city
 => "Bigville" # <-- delivery_address should have been updated, not address
2.5.3 :009 > acme.reload.delivery_address.city
 => "Smallville" # <-- delivery_address unchanged despite call to #update