如何在Rails中添加加入模型的字段總和？

Question

如何使用Rails 3.2和MySql 5.5添加來自加入模型的字段的總和？

比方說，我有模型是這樣的：

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 

我需要使用一些條件來選擇帳戶，並添加到所有存款帳戶的每個人的總和。

這可以通過SQL就像這樣：

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> 

我怎樣才能做到這一點使用Rails（使用LEFT JOIN另一種方式來達到同樣的效果。）？

我想是這樣的：

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

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

領域正在總結amount是operations表;所以活動記錄查詢也將在Operation模型上。應該定義mycondition以包含屬於特定帳戶的操作。

Answer 2

如果你需要做一個LEFT JOIN即使取回賬戶時，他們沒有操作記錄，您需要輸入的是聯接條件的東西，如：

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 

如果你願意分成兩個查詢這個，這是一個選項：

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 

---

編輯：如果你需要的帳戶情況，需要通過總和進行排序，一些額外的SQL將開始悄悄卜牛逼這樣的事情應該工作：

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