使用key的值作为同一哈希值

时间:2016-05-17 16:44:32

标签: ruby-on-rails ruby hash key

如何将一个键的值作为操作的一部分用于Ruby中相同哈希中另一个键的值,在声明它的那一刻?

我的意思是这样的:

purchase = {product:  'phone',
            quantity: 5,
            price: 120,
            total: self.quantity * self.price
           }

我认为这会很有用,例如,如果值quantity由外部API分配,其中消耗有限的查询,那么如果total请求,则会花费两个查询而不是询问为数量'只花一个查询。

2 个答案:

答案 0 :(得分:4)

我建议创建一个进行计算的Purchase模型:

class Purchase
  attr_reader :product, quantity, price

  def initialize(attributes = {})
    @product  = attributes[:product]
    @quantity = attributes[:quantity]
    @price    = attributes[:price]
  end

  def total 
    quantity * price
  end

  def to_h
    {
      product:  product,
      quantity: quantity,
      price:    price,
      total:    total
    }
  end
end

将代码更改为:

purchase = Purchase.new(product: 'phone', quantity: 5, price: 120).to_h

作为奖励:此模型易于理解且易于测试。

答案 1 :(得分:1)

在哈希宣言期间,您无法提出要求。您可以像这样更改代码:

purchase = {}
purchase[:product]  = 'phone'
purchase[:quantity] = 5
purchase[:price]    = 120
purchase[:total]    = purchase[:quantity] * purchase[:price]

或者只是最后一行:

purchase = {
 product: 'phone',
 quantity: 5,
 price:    120
}
purchase[:total] = purchase[:quantity] * purchase[:price]

或其他涉及稍微减少打字的愚蠢方式:

purchase = {
 product: 'phone',
 quantity: 5,
 price:    120
}.tap{ |h| h[:total] = h[:quantity] * h[:price] }

但是,我建议在这种情况下你不应该在你的哈希中存储"denormalized" data。由于total取决于数量或价格,如果其中任何一个更改,您的哈希将无效。

您可以通过在哈希上创建一个特殊的default_proc来解决这个问题,每次请求时都会计算总数:

purchase = {
 product: 'phone',
 quantity: 5,
 price:    120
}
purchase.default_proc = ->(h,k){
  h[:quantity]*h[:price] if k==:total
}
p purchase[:total]      #=> 600
purchase[:quantity] = 7
p purchase[:total]      #=> 840

但是,创建一个类或Struct来更清楚。结构代码的代码较少:

Purchase = Struct.new(:product,:quantity,:price) do
  def total
    quantity * price
  end
end
purchase = Purchase.new('phone',5,120)
p purchase.total                       #=> 600
purchase.quantity = 3
p purchase.total                       #=> 360

但是,Struct不会(by default)允许关键字参数。通过编写自己的类,您可以按任何顺序提供参数,甚至提供默认值:

class Purchase
  attr_reader :product, :price, :quantity
  def initialize(product:,price:,quantity:1) # default quantity
    @product, @price, @quantity = product, price, quantity
  end
  def total
    price*quantity
  end
end

Purchase.new(price:10, product:'shoes').total              #=> 10
Purchase.new(product:'shoes', price:10).total              #=> 10
Purchase.new(quantity:3, price:10, product:'shoes').total  #=> 30