Rails Rails 4 中的 through 与 scope 共存时的 includes 查询，这个坑如何跳
题目有点拗口。请忽略。
下面是正题：
现在我有 3 个 model，分别是 Pyysician、Appointment、Patient；他们的关系如下：
class Physician < ActiveRecord::Base
has_many :appointments
has_many :patients, through: :appointments
has_many :healed_patients, -> { where(state: Patient::HEALED).order('appointments.created_at DESC')}, through: :appointments, class_name: 'Patient'
end
class Appointment < ActiveRecord::Base
belongs_to :physician
belongs_to :patient
belongs_to :healed_patient, class_name: 'Patient', foreign_key: 'patient_id'
end
class Patient < ActiveRecord::Base
HEALED = 1
has_many :appointments
has_many :physicians, through: :appointments
end
如果现在有 Physician 的一个实例physician，用它调用physician.healed_patients是可以正常工作的。如果使用Physician.includes(:healed_patients).last，则会报错。
错误如下：
2.2.1 :001 > phy = Physician.last
Physician Load (0.1ms) SELECT "physicians".* FROM "physicians" ORDER BY "physicians"."id" DESC LIMIT 1
=> #<Physician id: 1, created_at: "2016-02-23 14:40:38", updated_at: "2016-02-23 14:40:38">
2.2.1 :002 > phy.healed_patients
Patient Load (0.2ms) SELECT "patients".* FROM "patients" INNER JOIN "appointments" ON "patients"."id" = "appointments"."patient_id" WHERE "appointments"."physician_id" = ? AND "patients"."state" = ? ORDER BY appointments.created_at DESC [["physician_id", 1], ["state", 1]]
=> #<ActiveRecord::Associations::CollectionProxy [#<Patient id: 1, state: 1, created_at: "2016-02-23 14:40:56", updated_at: "2016-02-23 14:40:56">]>
2.2.1 :003 > phy = Physician.includes(:healed_patients).last
Physician Load (0.2ms) SELECT "physicians".* FROM "physicians" ORDER BY "physicians"."id" DESC LIMIT 1
Appointment Load (0.7ms) SELECT "appointments".* FROM "appointments" WHERE "patients"."state" = ? AND "appointments"."physician_id" IN (1) [["state", 1]]
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: patients.state: SELECT "appointments".* FROM "appointments" WHERE "patients"."state" = ? AND "appointments"."physician_id" IN (1)
请问有什么方法解决 includes 的问题否 或者 我的这个写法哪里有问题？ 谢谢诸位。谢谢
报错说：no such column: patients.state`, 看看你的 db/schema.rb 里面，patients 表里面有 state 这个字段吗？是不是有 migration 没有运行？
#2 楼 @leomayleomay state 在 Physician 表上，如果没法使用 includes 实现，就手写 sql 语句实现，避免 N+1 问题也是可以的，只是难看了点
对比第二条和最后一条查询，你会发现第二条有 Join 操作，而最后一条是没有的，问题出在这里FROM "appointments" WHERE "patients"."state" = ?。
另外从代码语义上来说，感觉Physician.includes(:healed_patients).last 意图不明确，最后一位康复的病人？，还是有康复病人的最后一位医生？
详见https://ruby-china.org/topics/28251，希望可以帮到你。
-
Physician.includes(:healed_patients).last仅作问题测试之用，真实场景用 where 或 find 的。 - Thanks，链接我看下。
那个 block 似乎应该放到 patient 里面
class Physician < ActiveRecord::Base
has_many :appointments
has_many :patients, through: :appointments
has_many :healed_patients, through: :healed_appointments, class_name: 'Appointment'
end
class Appointment < ActiveRecord::Base
belongs_to :physician
belongs_to :patient
belongs_to :healed_patient, class_name: 'Patient', foreign_key: 'patient_id'
end
class Patient < ActiveRecord::Base
HEALED = 1
has_many :healed_appointments, -> { where(state: Patient::HEALED).order('appointments.created_at DESC')}, class_name: 'Appointment'
has_many :appointments
has_many :physicians, through: :appointments
end
看起来像是 ActiveRecord 的 bug
Physician.where_has_healed_patient.include(:patients).last # 找到有治愈病人的最后一个医生
其实语义好像是这个的样子 我理解 include 只是用来拿“一对多”的“多”的，你的写法没有体现“找到有治愈病人的医生”这个意思，直接读是“我要所有医生中的最后一个，带上他的治愈病人的信息”。
不是很清楚 rails 这里具体怎么实现，只是从语义上看。
或者你就是是要找第一个医生，并带上他的治愈病人信息？
#7 楼 @lithium4010 谢谢。
语义可以理解为：找到最后一个医生，和他治疗好的病人
您回复的内容是不是有问题？ where_has_healed_patient这个方法在Physician上不存在。
另外：
class Physician < ActiveRecord::Base
has_many :appointments
has_many :patients, through: :appointments
has_many :healed_patients, through: :healed_appointments, class_name: 'Appointment'
end
最后一句是不是写错了？