列trades.item_id爲什麼不存在？

Question

我有一個關係模型，其中兩個用戶可以進入交易兩個項目交換。

class User < ActiveRecord::Base 
has_many :owned_items, class_name: "Item" 
has_many :trades_received, class_name: "Trade", through: :owned_items, source: :trades 
has_many :trades 
has_many :wanted_items, class_name: "Item", through: :trades, source: :item 
end 

class Item < ActiveRecord::Base 
belongs_to :owner, class_name: "User", foreign_key: :user_id 
has_many :trades, dependent: :destroy 
has_many :trade_requesters, through: :trades 
has_many :trade_recipients, through: :trades 
end 

class Trade < ActiveRecord::Base 
belongs_to :trade_requester, class_name: "User" 
belongs_to :trade_recipient, class_name: "User" 
belongs_to :wanted_item, class_name: "Item", foreign_key: :wanted_item_id 
belongs_to :collateral_item, class_name: "Item", foreign_key: :collateral_item_id 
end 

遷移我的交易表看起來像這樣：

create_table :trades do |t| 
t.belongs_to :trade_requester 
t.belongs_to :trade_recipient 
t.belongs_to :wanted_item 
t.belongs_to :collateral_item 
end 

堆棧跟蹤導致我使用列出所有交易請求一個輔助方法。該行表示@trades = current_user.trades_received.requested.count，然後放到模型關聯的User上，其中has_many :owned_items, class_name: "Item"。根據我的理解，它看起來像trades_received方法，它被稱爲through: :owned_items和source: :trades應該引用:wanted_item_id外鍵進行遷移。但事實並非如此。如果我創建一個遷移來添加item_id，但它有效，但貿易需要兩個項目，所以我已將它分成兩個wanted_item和collateral_item關聯。如何設置該用戶關聯以便引用其他用戶請求的項目？應該項目has_many :trades，我的方式，或者應該項目belongs_to :trades？

完整的錯誤：

PG::UndefinedColumn: ERROR: column trades.item_id does not exist 
LINE 1: ...LECT COUNT(*) FROM "trades" INNER JOIN "items" ON "trades"."... 
                 ^
: SELECT COUNT(*) FROM "trades" INNER JOIN "items" ON "trades"."item_id" = "items"."id" WHERE "items"."user_id" = $1 AND "trades"."approved" IS NULL 

tldr：我需要跟蹤一堆複雜的has_many :through協會，我不認爲我的數據模型是正確的，需要幫助理解爲什麼。謝謝。

Answer 1

您正在設置User和Item之間的兩個has_many :through關係，並將Trade作爲兩者的連接表。你有一些關係困惑。這是根據您的遷移設置：

class User < ActiveRecord::Base 
has_many :received_trades, class_name: "Trade", foreign_key: "trade_recipient" 
has_many :requested_trades, class_name: "Trade", foreign_key: "trade_requester" 
has_many :collateral_items, through: :received_trades 
has_many :wanted_items, through: :requested_trades 
end 

class Item < ActiveRecord::Base 
has_many :collateral_items, class_name: "Trade", foreign_key: "collateral_item" 
has_many :wanted_items, class_name: "Trade", foreign_key: "wanted_item" 
has_many :trade_requesters, through: :wanted_items 
has_many :trade_recipients, through: :collateral_items 
end 

class Trade < ActiveRecord::Base 
belongs_to :trade_requester, class_name: "User" 
belongs_to :trade_recipient, class_name: "User" 
belongs_to :wanted_item, class_name: "Item" 
belongs_to :collateral_item, class_name: "Item" 
end 

##migration 
create_table :trades do |t| 
t.belongs_to :trade_requester 
t.belongs_to :trade_recipient 
t.belongs_to :wanted_item 
t.belongs_to :collateral_item 
end 

一些解釋：

Item has_many :collateral_item ## item_id in table collateral_items 
Item has_many :collateral_item, class_name: "Trade", foreign_key: "collateral_item" 
##collateral_item_id in trades table. 

Answer 2

好吧，好吧。問題就在這裏：

has_many :trades, dependent: :destroy 

而在你Trade型號：

belongs_to :wanted_item, ... 
belongs_to :collateral_item, .. 

的Rails不能自動處理這個問題。

你需要做的這個步驟之一（具體取決於您在您的應用程序的需要）：

如果需要單獨的關聯：

class User < ActiveRecord::Base 
    has_many :trades_received, class_name: "Trade", through: :owned_items, source: :wantable_trades 
end 

class Item < ActiveRecord::Base 
    has_many :wanted_trades, class_name: 'Trade', inverse_of: :wanted_item, dependent: :destroy 
    has_many :collateral_trades, class_name: 'Trade', inverse_of: :collateral_item, dependent: :destroy 
end 

如果你需要的所有交易單的關聯：

那麼，你會有一個痛苦的屁股:)在這種情況下，你應該手動選擇關聯，或重新考慮你的數據模型。