Rails 3.如何通過has_many關係使用reduce？

Question

我有這個代碼與shipment has one invoice一起使用，但現在我不得不將它更改爲shipment has many invoices。我如何修改以下代碼以反映新的關聯？

@totals = { 
    :overall => @shipments.reduce(0) { |total, shipment| total + shipment.invoice.customer_total }, 
    :paid => @shipments.reduce(0) { |total, shipment| total + shipment.invoice.customer_amount_paid }, 
    :balance => @shipments.reduce(0) { |total, shipment| total + shipment.invoice.customer_open_balance } 
    } 

Answer 1

我會做這樣的事情：

# collect all the invoices at once 
invoices = @shipments.map(&:invoices).flatten 

@totals = { 
    # collect all the customer_totals and sum them... repeat 
    :overall => invoices.map(&:customer_total).reduce(0, :+), 
    :paid => invoices.map(&:customer_amount_paid).reduce(0, :+), 
    :balance => invoices.map(&:customer_open_balance).reduce(0, :+) 
} 

注：Enumerable#reduce，與許多其他方法沿着Enumerable，有一個塊或符號的工作名稱的方法或者操作者的能力。這使您可以用[1,2,3,4,5].reduce(:+)替換[1,2,3,4,5].reduce{ |sum, x| sum + x }

另外，如果未給出參數，則將集合中的第一個值假定爲備忘錄的初始值。

正如tokland指出的那樣，如果數組爲空，您需要將備忘錄的初始值作爲0傳遞。這可以防止您獲得@totals[:balance] == nil。