Question

我有一个带有费用关联的部门模型：

class Department < ActiveRecord::Base
  has_many :expenses
end

class Expense < ActiveRecord::Base
  belongs_to :department
end

费用有金额属性：

e = Expense.new
e.amount = 119.50

我现在想要2个查询：

列出所有部门，按费用的总和排序。
与＃1相同，但按月分组，即jan，feb，march，...

Answer 1

对于＃1，以下代码将为您提供按费用总和排序的部门ID：

Expense.select('department_id, sum(amount) as total').group('department_id').order('total desc')

以下是有关如何使用返回对象的示例代码：

Expense.select('department_id, sum(amount) as total').group('department_id').order('total desc').each { |dep| print "Department ID: #{dep.department_id} | Total expense: #{dep.total}\n" }

这将打印如下内容：

部门ID：2 |总费用：119.50
部门编号：1 |总费用：54.34
部门ID：10 |总费用：23.43

对于＃2，您可以类似地添加月份分组以及总和：

Expense.select('department_id, extract(month from created_at) as month, sum(amount) as total').group('department_id, month').order('month asc, total desc')

再次，演示如何使用它的示例代码：

Expense.select('department_id, extract(month from created_at) as month, sum(amount) as total').group('department_id, month').order('month asc, total desc').each { |dep| print "Department ID: #{dep.department_id} | Month: #{dep.month} | Total expense: #{dep.total}\n" }

这将打印如下内容：

部门ID：2 |月：1 |总费用：119.50
部门编号：1 |月：1 |总费用：54.34
部门ID：10 |月：1 |总费用：23.43
部门编号：1 |月：2 |总费用：123.45
部门编号：2 |月：2 |总费用：76.54
部门ID：10 |月：2 |总费用：23.43

......等等。

当然，一旦你有了部门ID，你可以使用Department.find（）来获取其余的信息。我相信ActiveRecord不支持在不使用原始SQL的情况下直接同时获取所有Department字段。

编辑----

如果您想要包含部门字段，您可以：

1 - 将它们加载到单独的查询中，例如：

Expense.select('department_id, sum(amount) as total').group('department_id').order('total desc').each do |department_expense|
    # In department_expense you have :department_id and :total
    department = Department.find(department_expense.department_id)
    # In department now you have the rest of fields
    # Do whatever you have to do with this row of department + expense
    # Example
    print "Department #{department.name} from #{department.company}: $#{department_expense.total}"
end

优点：使用ActiveRecord SQL抽象非常简洁。
缺点：您正在进行总共N + 1个查询，其中N是部门数，而不是单个查询。

2 - 使用原始SQL加载它们：

Department.select('*, (select sum(amount) from expenses where department_id = departments.id) as total').order('total desc').each do |department|
    # Now in department you have all department fields + :total which has the sum of expenses
    # Do whatever you have to do with this row of department + expense
    # Example
    print "Department #{department.name} from #{department.company}: $#{department.total}"
end

优势：您正在执行单个查询。
缺点：您正在失去ActiveRecord从SQL提供给您的抽象。

两者都会打印出来：

来自微软的部门R＆amp; D：119.50美元雅虎的部门财务部门：54.34美元 Google部门设施：$ 23.43

按协会财产的总和排序

1 个答案: