Rails belongs_to 通过关联

Question

我正在尝试将新模型添加到现有模型网格中。现有的工作完美，但我无法让新的工作正常，我想知道该协会是否能够按照我试图使其工作的方式工作。更新：正如我刚刚被问到的那样：belongs_to through 是我在解决这个问题时读到的。如果它不存在，has_one through 会是正确的方法吗？我也试过了，还是不行。

这是现有的网格：

class Course
  has_many :users, through: :enrollments
  has_many :enrollments
end

class User
  has_many :courses, through: :enrollments
  has_many :enrollments
end

class Enrollment
  belongs_to :course
  belongs_to :user

  # has fields :user_id, :course_id
end

现在用户应该能够对他已完成的课程进行评分。 （如果他有，有一个带有他的 id 和课程 id 的注册。）我认为最好这样写：

class Course
  has_many :users, through: :enrollments
  has_many :enrollments
  has_many :ratings, through: :enrollments
end

class User
  has_many :courses, through: :enrollments
  has_many :enrollments
  has_many :ratings, through: :enrollments
end

class Enrollment
  belongs_to :course
  belongs_to :user
  has_one :rating

  # has fields :user_id, :course_id
end

class Rating
  belongs_to :enrollment
  belongs_to :course, through: :enrollment
  belongs_to :user, through: :enrollment
end

当我尝试在控制台中创建评级时，我收到以下错误：

User.first.ratings.create(text: "test", course_id: Course.first.id)
ArgumentError: Unknown key: through

更新 当我使用has_one through insted 时，出现以下错误：

ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection: Cannot modify association 'User#ratings' because the source reflection class 'Rating' is associated to 'Enrolment' via :has_one.

完全可以这样做吗？谢谢！

Answer 1

class Course
  has_many :users, through: :enrollments
  has_many :enrollments
  has_many :ratings, through: :enrollments
end

class User
  has_many :courses, through: :enrollments
  has_many :enrollments
  has_many :ratings, through: :enrollments
end

class Enrollment
  belongs_to :course
  belongs_to :user
  belongs_to :rating

  # has fields :user_id, :course_id, rating_id
end

class Rating
  has_one :enrollment
  has_one :course, through: :enrollment
  has_one :user, through: :enrollment
end

注意：添加外键列

如果您在评级表中只有一/两列，请将它们合并到这样的注册中。

class Course
  has_many :users, through: :enrollments
  has_many :enrollments
end

class User
  has_many :courses, through: :enrollments
  has_many :enrollments
end

class Enrollment
  belongs_to :course
  belongs_to :user

  # has fields :user_id, :course_id, rating-columns...
end

Answer 2

结构

也许你太复杂了

class Enrollment
  belongs_to :course
  belongs_to :user
end

这意味着您有一个存储唯一记录的连接模型，引用course 和user。您的ratings 是基于user 和course 的吗？

为什么不将rating 作为enrolment 模型的属性包含在内？：

#enrolments
id | user_id | course_id | rating | created_at | updated_at

如果您给 rating 一个数值 (1 - 5)，它将使您能够像这样对不同的注册进行评分：

user = User.first
course = Course.first

user.enrolments.create(course: course, rating: 5)

--

评分

当然，这是基于您当前的模型结构。

如果您想通过users 为courses 包含ratings（不绑定到enrolment），您可能希望使用称为join model 的join model 或类似名称：

#app/models/user.rb
Class User < ActiveRecord::Base
   has_many :enrolments
   has_many :courses, through: :enrolments
   has_many :ratings, through: :courses, class_name: "CourseRating"
end

#app/models/course.rb
Class Course < ActiveRecord::Base
   has_many :enrolments
   has_many :students, through: :enrolments, class_name: "User"
   has_many :ratings, class_name: "CourseRating"
end

#app/models/course_rating.rb
Class CourseRating < ActiveRecord::Base
   belongs_to :course
   belongs_to :user
end

这将允许您调用：

user = User.first
user.courses.first.ratings