按协会财产的总和排序

Question

我有一个费用协会的部门模型:

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:

1. 列出所有部门,按费用的总和排序.

2. 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:

部门编号:2 | 总费用:119.50
部门ID: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:

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

优点:您正在进行单个查询.
缺点:您正在失去ActiveRecord从SQL提供给您的抽象.

两者都会打印:

微软部门研发
部门:雅虎部门财务部门119.50 美元
:谷歌部门设施54.34 美元:23.43美元