如何通过Rails 4中的关联使用ActiveRecord has_many从特定表中检索记录？

Question

使用来自RailsGuide的has_many：through关联示例，如何使用ActiveRecord选择没有预约的所有患者或医生？

---

2.4 has_many：通过协会 http://guides.rubyonrails.org/association_basics.html#the-has-many-through-association

class Physician < ActiveRecord::Base
  has_many :appointments
  has_many :patients, through: :appointments
end

class Appointment < ActiveRecord::Base
  belongs_to :physician
  belongs_to :patient
end

class Patient < ActiveRecord::Base
  has_many :appointments
  has_many :physicians, through: :appointments
end

---

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

    has_many :enrollments
    has_many :courses, through: :enrollments

end

class Enrollment < ActiveRecord::Base
    belongs_to :course
    belongs_to :user

    scope :registered, -> { where("enrolled = ?", true) }
    scope :student, -> (student_id) { where("user_id = ?", student_id) }
end

class Course < ActiveRecord::Base
    before_save :update_slug

    has_many :enrollments
    has_many :users, through: :enrollments

    def update_slug
        self.slug = title.parameterize
    end

    def to_param
        slug
    end

    # scope :available, -> { where.not(id: Enrollment.pluck(:course_id)).uniq }
    scope :available, -> (student_id) { where.not(id: Enrollment.pluck(:course_id)).uniq }
end

Answer 1

您可以做的最简单的事情是，首先从联接表/模型中选择所有ID，然后使用该列表从第二个查询中排除记录：

Physician.where.not(id: Appointment.pluck(:physician_id).uniq)
Patient.where.not(id: Appointment.pluck(:patient_id).uniq)

Answer 2

您也可以使用我个人更喜欢的纯SQL：

Physician.where("
  not exists (select 1 from appointments 
  where appointments.physician_id = physician.id)
")

Patient("
  not exists (select 1 from appointments 
  where appointments.patient_id = patient.id)
")

至于“我该如何根据签名的患者来过滤记录？”，那是什么意思？

Appointment.find_by(patient_id: 123).patient
Appointment.find_by(patient_id: 123).physician