多对多关系：通过给出“找不到关联”错误答案

【问题标题】：Many to Many relationship :through giving 'Could not find the association' error多对多关系：通过给出“找不到关联”错误
【发布时间】：2013-05-17 02:14:21
【问题描述】：

在我的模型中，Item 是由 User 创建的，可以被许多 Users 购买，User 可以购买许多 Items。

User、Item 和 Purchase 是使用 AcvtiveRecord 定义的为简洁起见，省略了多余的细节，如下所示：

class User < ActiveRecord::Base
  # various other fields
  has_many :items, :foreign_key => :creator_id
  has_many :purchased_items, :through => :purchases, :source => :item
end

class Item < ActiveRecord::Base
  # various other fields
  belongs_to :creator, :class_name => 'User'
  has_many :buyers, :through => :purchases, :source => :user
 end

 class Purchase < ActiveRecord::Base
   belongs_to :item
   belongs_to :user
  # various other fields
 end

还有一个rspec 测试也截断了如下：

describe "user purchasing" do
  it "should allow a user to purchase an item" do
    a_purchase = Purchase.create!(:item => @item, # set up in `before :each`
                                  :user => @user  # set up in `before :each`
    )
    a_purchase.should_not eq(nil)                 # passes
    @item.buyers.should include @user             # fails
    @user.purchased_items.should include @item    # fails
  end
end

这会导致

1) Purchase user purchasing should allow a user to purchase an item
   Failure/Error: @item.buyers.should include @user
   ActiveRecord::HasManyThroughAssociationNotFoundError:
     Could not find the association :purchases in model Item

同样，如果我交换 @file_item.buyers.should include @user 和 @user.purchased_items.should include @item 我会得到等价的

1) Purchase user purchasing should allow a user to purchase an item
   Failure/Error: @user.purchased_items.should include @item
   ActiveRecord::HasManyThroughAssociationNotFoundError:
     Could not find the association :purchases in model User

我的migration 看起来像

create_table :users do |t|
  # various fields
end

create_table :items do |t|
  t.integer :creator_id   # file belongs_to creator, user has_many items
  # various fields
end

create_table :purchases do |t|
  t.integer :user_id
  t.integer :item_id
  # various fields
end

我做错了什么？

【问题讨论】：

如果你在 User 和 Item 中删除 , :source => :item 会发生什么？
好问题。我只是尝试过，这并没有什么不同。我仍然得到Could not find the association :purchases in model Item（反之亦然）。

标签： ruby activerecord has-many-through

【解决方案1】：

您必须指定以下内容。

class User < ActiveRecord::Base
  has_many :purchases
  has_many :items, :foreign_key => :creator_id
  has_many :purchased_items, :through => :purchases, :source => :item
end

class Item < ActiveRecord::Base
  # various other fields
  has_many :purchases
  belongs_to :creator, :class_name => 'User'
  has_many :buyers, :through => :purchases, :source => :user
end

仅当您指定时

      has_many :purchases

模型将能够识别关联。

【讨论】：