如何在Rails中加入模型的字段总和？

Question

如何使用Rails 3.2和MySql 5.5从联接的模型中添加字段的总和？

假设我有这样的模型：

class Account < ActiveRecord::Base
  attr_accessible :number
  has_many :operations
end

class Operation < ActiveRecord::Base
  belongs_to :account
  attr_accessible :op_type,  # either 'deposit' or 'withdrawal'
                  :amount
end

我需要使用某种条件选择帐户，并将该帐户所有存款的总和加到每个帐户中。

可以使用以下SQL来完成：

SELECT *,
    IFNULL((
        SELECT SUM(amount)
        FROM operations
        WHERE operations.account_id = accounts.id AND operations.op_type = 'deposit'
    ), 0) as total_deposits
FROM accounts
WHERE <condition for accounts>

（使用LEFT JOIN是获得相同结果的另一种方法。）

如何使用Rails做到这一点？

我想要这样的东西：

accounts = Account.where(<mycondition>). join(???). sum(???)  # What should be here?
accounts.each do |a|
  puts "Account #{a.number} has deposited #{a.total_deposits} total."
end

Answer 1

尝试Operation.joins(:account).where(<mycondition>).sum(:amount)

外地被求和amount是在operations表; 因此，活动记录查询也将在Operation模型上进行。 mycondition应该定义为包括属于特定帐户的操作。

Answer 2

如果即使没有操作记录，也需要执行LEFT JOIN来检索帐户，则需要使用以下命令键入该连接条件：

totals = Account.where(<account conditions>).joins("LEFT JOIN operations ON operations.account_id = accounts.id AND operations.op_type = 'deposit'").group("accounts.number").sum(:amount)
totals.each do |a|
  puts "Account #{a[0]} has deposited #{a[1]} total."
end

如果您愿意将此分为两个查询，则可以选择以下方法：

accounts = Account.where(<account conditions>)
totals = Operation.where(op_type: "deposit", account_id: accounts.map(&:id)).group(:account_id).sum(:amount)
accounts.each do |a|
  puts "Account #{a.number} has deposited #{totals[a.id] || 0} total."
end

---

编辑：如果您需要帐户实例并需要按总和进行排序，则将开始引入一些其他SQL。但是类似这样的方法应该起作用：

accounts = Account.where(<account conditions>).joins("LEFT JOIN operations ON operations.account_id = accounts.id AND operations.op_type = 'deposit'").group("accounts.number").select("accounts.*, COALESCE(SUM(amount), 0) AS acct_total").order("acct_total")
accounts.each do |a|
  puts "Account #{a.number} has deposited #{a.acct_total} total."
end