在联接表-Rails中保存值

我正在尝试将amount值添加到联接表中。以下是我的模特:

class Invoice
    has_many :invoice_payments
    has_many :payments, through: :invoice_payments
end

class InvoicePayment
    belongs_to :invoice
    belongs_to :payment
end

class Payment
    has_many :invoice_payments
    has_many :invoices, through: :invoice_payments 
end

我想联系 N 单笔付款的发票数。对于每一张发票,我想在join表中保存正在支付的金额。

这是我的控制器代码:

 payment = Payment.new
  if params.has_key?(:invoices)
    params[:invoices].each do |i|


        invoice = Invoice.find(i[:id])
        invoice.amount_paid = calculate_amount_paid


       payment.invoices << invoice
    end
  end

如您所见,我不知道如何在InvoicePayment表上设置金额。该字段在此表上不存在。提前谢谢。