这个 has_many :through 关系是否应该是多态的？

Question

我通过has_many :through 为Teachers 和Classrooms 设置了多对多关系：

class Teacher < ActiveRecord::Base
  has_many :classrooms, :through => :classroom_memberships
end

class Classroom < ActiveRecord::Base
  has_many :students
  has_many :teachers, :through => :classroom_memberships
end

class ClassroomMemberships < ActiveRecord::Base
  belongs_to :teacher
  belongs_to :classroom
end

目前Students只能属于一个Classroom：

class Student < ActiveRecord::Base
  belongs_to :classroom
end

现在我需要跟踪学生的历史课堂成员资格，为课堂创建第二个多对多关系。因此，虽然一个学生一次只能属于一个教室，但我需要知道去年学生 A 属于 B 教室。

我认为我有两个可行的选择：

1.) 将classroom_memberships 关联设为多态，这样我就有了classroomable_id 和classroomable_type，它们将指向老师或学生。

2.) 简化事情并向 ClassroomMemberships 添加另一个名为 student_id 的外键，在这种情况下，对于给定的行，student_id 或 teacher_id 都会有一个值。

哪个是更好的选择？

Answer 1

我可能会走：

class Course < ActiveRecord::Base
  # like "MATH 100"
  has_many :sections
  has_many :teachers, :through => :sections
end

class Term < ActiveRecord::Base
  # like "Fall 2015"
  has_many :sections
end

class Teacher < ActiveRecord::Base
  has_many :sections
  has_many :courses, :through => :sections
end

class Section < ActiveRecord::Base
  # a course, in a term, taught by a teacher, with registered students
  belongs_to :term
  belongs_to :course
  belongs_to :teacher
  has_many :registrations
  has_many :students, :through => :registrations
end

class Registration < ActiveRecord::Base
  # a student in a specific section
  belongs_to :section
  belongs_to :student 
end

class Student < ActiveRecord::Base
  # a student's registrations are their course history
  has_many :registrations
  has_many :sections, through :registrations
end

首先，因为这是一个相当基本的教育系统建模。

Answer 2

听起来您可能想要一个 ClassroomMembershipHistory 模型。

类似

class ClassroomMembershipHistory < ActiveRecord::Base
  belongs_to :student
  belongs_to :classroom
end

带有year 属性，但存储起来最容易查询您的用例。