Datamapper DataMapper

When I use go-through association like this, there will be a error.

class Person
  include DataMapper::Resource
  property :id, String, :key => true, :length => 20
  property :created_at, Time
  has n, :loans, :child_key => [:loaner_id], :constraint => :protect
  has n, :debts, "Loan", :child_key => [:loanee_id], :constraint => :protect
  has n, :loan_creations, :child_key => [:loaner_id], :constraint => :protect
  has n, :debt_creations, "LoanCreation", :child_key => [:loanee_id], :constraint => :protect
  has n, :loan_deletions, :through => :loans
  def potential_loans
    loan_creations.undone
  end
  def potential_debts
    debt_creations.undone
  end
end


class LoanDeletion
  include DataMapper::Resource
  belongs_to :loan, :key => true
  def self.undone
    all(:loan => {:status => :open})
  end
  def self.done
    all(:loan => {:status => :closed})
  end
end

Error message:

in `finalize_model': LoanDeletion must have a key to be valid (DataMapper::IncompleteModelError)

But if I give LoanDeletion model a id in Serial, the error will be gone.

class LoanDeletion
  include DataMapper::Resource
  belongs_to :loan, :key => true
  property :id, Serial
  def self.undone
    all(:loan => {:status => :open})
  end
  def self.done
    all(:loan => {:status => :closed})
  end
end