列trades.item_id为什么不存在？

Question

我有一个关系模型，其中两个用户可以进入交易两个项目交换。

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 

迁移我的交易表看起来像这样：

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 

堆栈跟踪导致我使用列出所有交易请求一个辅助方法。该行表示@trades = current_user.trades_received.requested.count，然后放到模型关联的User上，其中has_many :owned_items, class_name: "Item"。根据我的理解，它看起来像trades_received方法，它被称为through: :owned_items和source: :trades应该引用:wanted_item_id外键进行迁移。但事实并非如此。如果我创建一个迁移来添加item_id，但它有效，但贸易需要两个项目，所以我已将它分成两个wanted_item和collateral_item关联。如何设置该用户关联以便引用其他用户请求的项目？应该项目has_many :trades，我的方式，或者应该项目belongs_to :trades？

完整的错误：

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：我需要跟踪一堆复杂的has_many :through协会，我不认为我的数据模型是正确的，需要帮助理解为什么。谢谢。

Answer 1

您正在设置User和Item之间的两个has_many :through关系，并将Trade作为两者的连接表。你有一些关系困惑。这是根据您的迁移设置：

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 

一些解释：

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。问题就在这里：

has_many :trades, dependent: :destroy 

而在你Trade型号：

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

的Rails不能自动处理这个问题。

你需要做的这个步骤之一（具体取决于您在您的应用程序的需要）：

如果需要单独的关联：

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单的关联：

那么，你会有一个痛苦的屁股:)在这种情况下，你应该手动选择关联，或重新考虑你的数据模型。