# 价格字段的单位转换

我们在存储一个产品的价格时，字段类型的设计可以有以下几种：

* Decimal - 以元为单位存储浮点数
* Integer - 以分为单位储存整数

如果用浮点数存储数据，那么我们在表单中录入价格时，时常会遇到一个奇怪的现象，录入的价格是`912.00`元，而存储数据库时莫名其妙地变成了`911.999999999999` 。呈现到前端时，就很怪异了。

在ruby中我可以通过下面测试，发现同样问题：

```ruby
2.3.4 :017 > "9.12".to_f * 100
 => 911.9999999999999
```

因此，我们就有了以分为单位存储的想法。但是，以分为单位，我们在生成表单时，又想让用户以元为单位录入价格，我们可能会这么做：

```bash
rails g scaffold product name price_cents:integer
```

{% code title="app/models/product.rb" %}

```ruby
class Product < ApplicationRecord
    # 数据库存储价格字段是 price_cents，整形，单位为分
    def price
        (price_cents || 0) / 100.0
    end

    def price=(v)
        # 这里用.round就是为了解决上面的911.99999999问题
        self.price_cents = (v.to_f * 100).round
    end
end
```

{% endcode %}

{% code title="app/controllers/products\_controller.rb" %}

```ruby
def product_params
    params.require(:product).permit(:name, :price)
end
```

{% endcode %}

{% code title="app/views/products/\_form.html.erb" %}

```markup
  <div class="field">
    <%= form.label :price %>
    <%= form.text_field :price, id: :product_price %>
  </div>
```

{% endcode %}

## 封装一下代码，做到足够DRY

{% code title="app/models/application\_record.rb" %}

```ruby
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  class << self
    def price_attr(model_attr, db_attr="#{model_attr}_cents")
      class_eval <<-CODE, __FILE__, __LINE__ + 1
        def #{model_attr}
          (#{db_attr} || 0) / 100.0
        end

        def #{model_attr}=(v)
          self.#{db_attr} = (v.to_f * 100).round
        end
      CODE
    end
  end
end
```

{% endcode %}

{% code title="app/models/product.rb" %}

```ruby
class Product < ApplicationRecord
    price_attr :price
end
```

{% endcode %}

以上代码，足够应付我们的价格前后端单位转换问题。如果需要更高级的功能，我们可以尝试Gem [money](https://github.com/RubyMoney/money) 或 [money-rails](https://github.com/RubyMoney/money-rails)

```ruby
require 'money'

# 10.00 USD
money = Money.new(1000, "USD")
money.cents     #=> 1000
money.currency  #=> Currency.new("USD")

# Comparisons
Money.new(1000, "USD") == Money.new(1000, "USD")   #=> true
Money.new(1000, "USD") == Money.new(100, "USD")    #=> false
Money.new(1000, "USD") == Money.new(1000, "EUR")   #=> false
Money.new(1000, "USD") != Money.new(1000, "EUR")   #=> true

# Arithmetic
Money.new(1000, "USD") + Money.new(500, "USD") == Money.new(1500, "USD")
Money.new(1000, "USD") - Money.new(200, "USD") == Money.new(800, "USD")
Money.new(1000, "USD") / 5                     == Money.new(200, "USD")
Money.new(1000, "USD") * 5                     == Money.new(5000, "USD")

# Unit to subunit conversions
Money.from_amount(5, "USD") == Money.new(500, "USD")  # 5 USD
Money.from_amount(5, "JPY") == Money.new(5, "JPY")    # 5 JPY
Money.from_amount(5, "TND") == Money.new(5000, "TND") # 5 TND

# Currency conversions
some_code_to_setup_exchange_rates
Money.new(1000, "USD").exchange_to("EUR") == Money.new(some_value, "EUR")

# Formatting (see Formatting section for more options)
Money.new(100, "USD").format #=> "$1.00"
Money.new(100, "GBP").format #=> "£1.00"
Money.new(100, "EUR").format #=> "€1.00"
```


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