.在ActiveRecord中查找方法
我正在使用 ActiveRecord 和 Ruby（Rails 之外）来跟踪一些统计数据。
我向其中一个模型添加了一个方法 (total_cost)，以使用当前模型中的几列以及另一个模型中的一列进行计算。
我真的很希望能够使用 ActiveRecord 的一些数学规定（求平均值、求和）以及使用我定义的方法进行简单的旧发现，但是任何这样做的尝试都会导致我（完全可以理解）“未知列”错误，例如
Article.find(:all, :conditions => ["total_cost > ?", 300])
结果，我正在以一种我们可以描述为暴力的方式做事，只需找到所有文章，然后将每个文章的total_value 填充到一个数组中，然后对其进行求和和平均值。
除了我正在做的事情之外，我还有其他选择吗？我是否应该通过 ActiveRecord 查看 MySQL 本身来计算有问题的值？
I'm using ActiveRecord and Ruby (outside of Rails) to track some statistics.
I added a method (total_cost) to one of my models to do calculations using a couple of the columns in the current model as well as a column from another model.
I'd really like to be able to use some of ActiveRecord's provisions for math (averaging, sums) and plain old finding using the method I defined, but any attempt to do so nets me (a perfectly understandable) 'Unknown column' error, e.g.
Article.find(:all, :conditions => ["total_cost > ?", 300])
As a result, I'm doing things in a manner we could describe as brute-forcey, just finding all the Articles then stuffing each one's total_value into an array and doing sums and averages with that.
Do I have any alternatives besides what I'm doing? Should I be looking past ActiveRecord to MySQL itself to calculate the values in question?
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你可以做
给定total_cost是一个方法：
You can do
Given total_cost is a method:
一种选择是将总成本的计算方式转换为 SQL 表达式，以便您可以将该表达式替换为查询中可能需要的“total_cost”的任何引用。如果总成本计算起来很复杂或者取决于多个表格，那么这可能会非常麻烦。
如果可行的话，在您的
Article模型上设置total_cost属性听起来确实有意义。您可以在模型更改时使用回调进行计算，然后保存，而不是在有人调用方法a.total_cost时进行计算。例如：这将允许您使用总和并平均总成本。这比获取文章、计算每篇文章的总成本然后汇总它们要高效得多。它可能也比每次请求时计算它更有效。
该解决方案取决于总成本值不会发生太大变化。如果它根据各种不同对象的变化而一直变化，那么检测总成本何时必须变化将会更加困难，并且以这种方式计算它的效率会降低。但这肯定会让你的计算变得更容易。
One option is to translate the way that total cost is calculated into an SQL expression so that you can then substitute that expression for any reference you might need to 'total_cost' in a query. That could potentially be quite cumbersome if total cost is complicated to calculate or depends on several tables.
If it is feasible it does sound like it makes sense to have a
total_costattribute on yourArticlemodel. Instead of having the calculation take place when someone calls the methoda.total_costyou could calculate it when the model changes using a callback and save it then. For example:That would allow you to use the sum and average the total costs. It would be much more efficient than getting articles, calculating the total cost for each and then aggregating them. It would probably also be more efficient than calculating it each time it is requested.
That solution is dependent on the value for total cost not changing a lot. If it changes all the time based on various different objects changing then it will be more awkward to detect when total cost must change and less efficient to calculate it in this way. It would certainly make your calculations easier though.
find方法创建 SQL。您不能在其中使用total_cost方法。所以需要用SQL编写total_cost的计算。如果您想使用该方法，您可以从所有数据中过滤必要的数据。但这种方式花费更多的资源..
findmethod creates SQL. You can't usetotal_costmethod in it. So it needs to write the calculation oftotal_costin SQL. If you want to use the method, you can filter necessary data from all.But this way spends more resources..