Question

我在我的Rails应用程序中的select语句中有这个：

...
 #<DailyStat subject: "Macroeconomics", topic: "Overview Of Economics", item_count: 160, correct_item_count: 0>,
 #<DailyStat subject: "Macroeconomics", topic: "Overview Of Economics", item_count: 1, correct_item_count: 0>,
 #<DailyStat subject: "Macroeconomics", topic: "Unemployment", item_count: 1, correct_item_count: 0>,
 #<DailyStat subject: "Macroeconomics", topic: "Inflation", item_count: 2, correct_item_count: 0>,...

当我对这些列进行基本选择时，我得到了这个。现在我的问题是，当我daily_stats.select("subject, topic, sum(correct_item_count), sum(item_count)").group(:topic)聚合列以组合item_counts和correct_item_counts时，结果结果集为：

#<DailyStat topic: "Demand And Consumer Behavior">, #<DailyStat topic: "Obligations">, #<DailyStat topic: "Computerized Auditing">, #<DailyStat topic: "Inflation">, #<DailyStat topic: "Overview Of Economics">

我输了数。这必须只是某种语法问题，但我遇到了麻烦。如何计算加起来的数量？

Answer 1

为了在select子句中包含subject，还需要将其添加到group子句中。

daily_stats.select(
  "subject, topic, sum(correct_item_count), sum(item_count)"
).group('topic, subject')

执行此查询时，您会看到输出为：

[#<DailyStat topic: "Demand And Consumer Behavior">, #<DailyStat topic: "Obligations">, #<DailyStat topic: "Computerized Auditing">, #<DailyStat topic: "Inflation">, #<DailyStat topic: "Overview Of Economics">]

您没有看到sum(correct_item_count)和sum(item_count)的原因，因为它们不是您模型的属性。

如果检查结果数组中某个对象的属性，您将看到其中的计数，但没有键。钥匙将是空的。

尝试：

daily_stats.select(
  "subject, topic, sum(correct_item_count), sum(item_count)"
).group('topic, subject').first.attributes

为简单起见，您可以为sum(...)添加别名。如下所示：

daily_stats.select(
  "subject, topic, sum(correct_item_count) as total_correct_item_count, sum(item_count) as total_item_count"
).group('topic, subject')

此查询会在您生成的每个对象上为您提供total_correct_item_count和total_item_count属性，以便您可以按以下方式访问它们：

daily_stats.select(
  "subject, topic, sum(correct_item_count) as total_correct_item_count, sum(item_count) as total_item_count"
).group('topic, subject').first.total_correct_item_count

Answer 2

别名计数。它们将成为返回对象的方法。

foos = foos.select("count(name) AS name_count").group(:bar)

foos.first.name_count # => something

当你没有为计数添加别名时，Rails可能会使用你的数据库所做的列名，但是如果确实如此，我也不想将它用作方法名。

Rails Active Record`group`：为什么这只返回几列？

2 个答案: