mongoid (4.0.0): df7f1fdd419c8613d53952992d9f5d233292db36
moped (1.5.0)
origin (1.1.0)
rails (4.0.0.rc1)

class User
  include Mongoid::Document
  include Mongoid::Timestamps  
  has_many :payments, inverse_of: :to
end

class Payment
  include Mongoid::Document
  include Mongoid::Timestamps

  field :amount, type: Integer

  belongs_to :to, polymorphic: true
  belongs_to :from, polymorphic: true
end

u = User.first
u.payments.create(from: Organization.first, amount: -1)
=> #<Payment _id: 519c71c6dc112ac376000001, amount: -1, to_type: nil, to_id: "519b1e4ddc112a0da7000001", from_type: "Organization", from_id: "519b1ea1dc112a2243000004">