简单的 Rails 嵌套关联 where 父子句的问题答案

【问题标题】：Trouble with simple Rails nested associations where clause for parent简单的 Rails 嵌套关联 where 父子句的问题
【发布时间】：2018-09-09 13:54:44
【问题描述】：

我有以下型号：

class Business < ApplicationRecord
  has_many :shopping_trips
end

class ShoppingTrip < ApplicationRecord
  belongs_to :business
  has_many :purchases
end

class Purchase < ApplicationRecord
  belongs_to :shopping_trip
end

因此，一个企业可以进行多次购物，而每次购物都可以进行多次购买。

我正在尝试对 Purchase 表运行一个简单的查询，以查找属于特定企业的购买。所以我写这个：

purchases = Purchase.joins(:shopping_trip => :business ).where(:shopping_trip => {:business_id => 1})

不幸的是，它不起作用。我收到以下错误：

ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "shopping_trip"
LINE 1: ...sses"."id" = "shopping_trips"."business_id" WHERE "shopping_...
                                                             ^
: SELECT "purchases".* FROM "purchases" INNER JOIN "shopping_trips" ON "shopping_trips"."id" = "purchases"."shopping_trip_id" INNER JOIN "businesses" ON "businesses"."id" = "shopping_trips"."business_id" WHERE "shopping_trip"."business_id" = $1

连接看起来正确，但 where 子句似乎失败了。

【问题讨论】：

标签： ruby-on-rails rails-activerecord

【解决方案1】：

ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR: missing 表“shopping_trip”的 FROM 子句条目

您需要在where 中指定表名而不是关联名。所以shopping_trip 应该是shopping_trips

purchases = Purchase.joins(:shopping_trip => :business ).where(:shopping_trips => {:business_id => 1})

【讨论】：

【解决方案2】：

更好的解决方案是设置间接关联，这样就可以通过join模型进行查询，而无需手动join：

class Business < ApplicationRecord
  has_many :shopping_trips
  has_many :purchases, through: :shopping_trips
end

class ShoppingTrip < ApplicationRecord
  belongs_to :business
  has_many :purchases
end

class Purchase < ApplicationRecord
  belongs_to :shopping_trip
  has_one :business, through: :shopping_trip
end

您现在可以从任一方查询：

@business = Business.eager_load(:purchases).find(1)
@purchases = @business.purchases

# or
@purchases = Purchase.eager_load(:buisness).where(businesses: { id: 1 })

【讨论】：

【解决方案3】：

检查这个...

all_purchase = 购买.all

all_purchase.each 做 |each_purchase| each_purchase.shopping_trip.business 结束

【讨论】：

这如何回答这个问题？