我如何比较与ActiveRecord的小数领域一个BigDecimal？ [英] How can I compare a BigDecimal with ActiveRecord decimal field?

问题描述

假设这样一个模式：

create_table "bills", :force => true do |t|
  t.decimal  "cost", :precision => 10, :scale => 5
end

我想编写一个函数，写一个新的法案，DB当且仅当它是独一无二的。下面不工作：

I want to write a function that writes a new bill to the DB iff it is unique. The following does not work:

def load_bill_unless_exists(candidate)
  incumbents = Bill.scoped.where(:cost => candidate.cost)
  candidate.save unless incumbents.exists?
end

由于现任票据和票据的候选人在他们的BigDecimal重新presentation不同的限制，因此：成本=＆GT; candidate.cost 测试失败。也就是说，它的比较

because the incumbent bills and the candidate bills have different limits in their BigDecimal representation, so the :cost => candidate.cost test fails. That is, it's comparing:

candidate: #<Bill id: nil, cost: #<BigDecimal:105e39850,'0.1670576666 6666666E4',27(27)>>

与

incumbent: #<ServiceBill id: 198449, cost: #<BigDecimal:105e35840,'0.167057667E4',18(18)>>

注意应聘者的BigDecimal的再$ P $比现任更多的数字psents成本。

Notice the candidate's BigDecimal represents the cost with more digits than the incumbent.

所以，问题很简单：什么是正确的方式来执行这种比较？我想到：成本=＆GT; BigDecimal.new（candidate.cost.to_s，18），但是感觉不对 - 例如，哪里是18号来自

So the question is simple: What's the right way to perform this comparison? I contemplated :cost => BigDecimal.new(candidate.cost.to_s, 18), but that doesn't feel right -- for example, where does that number 18 come from?

推荐答案

尝试使用的BigDecimal＃圆：

def load_bill_unless_exists(candidate)
  incumbents = Bill.scoped.where(:cost => candidate.cost.round(5))
  candidate.save unless incumbents.exists?
end

从文档：

取整到最近的1（默认），返回结果为一个BigDecimal。如果n被指定和正时，结果的小数部分具有不多于许多数字多

Round to the nearest 1 (by default), returning the result as a BigDecimal. If n is specified and positive, the fractional part of the result has no more than that many digits.

既然你已经指定了5 precision您的模式，这就是你应该做四舍五入比较时。

Given that you've specified a precision of 5 in your schema, that's what you should be rounding to when doing comparisons.

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