将属性从关联表注入到关联记录中？

Question

I have the following model in my Rails app:

class Course < ActiveRecord::Base
  has_many :memberships
  has_many :members, through: :memberships, class_name: 'User'
end

While course.members successfully returns the course's members, I don't get access to the Membership model which has a role attribute.

How do I find the user role without having to find the Membership given my Course and User ? Can I inject the role attribute to User somehow in the context association?

User model:

class User < ActiveRecord::Base
  has_many :memberships, dependent: :destroy
  has_many :courses, through: :memberships
end

Answer 1

Here's a solution, not the most beautiful and idiomatic though

class Course < ActiveRecord::Base
  has_many :memberships

  def members
    User.joins(:memberships).where(id: memberships.ids).select('users.*, memberships.role')
  end
end

A better approach, suggested through the comments:

has_many :members, -> { select('users.*, memberships.role') }, class_name: 'User', through: :memberships, source: :user

Answer 2

I had the exact same problem, and couldn't find any way to fetch the intermediate association without explicitly finding it. It's not that ugly, though:

class User < ActiveRecord::Base
  has_many :memberships, dependent: :destroy
  has_many :courses, through: :memberships

  def role_in(course)
    memberships.find_by!(course: course).role
  end
end

user.role_in(course)
# => "teacher"

Answer 3

I think dynamically storing the contextual role depending on what Course is used is not a good idea. It feels hacky to me because the attribute value becomes set implicitly rather than by explicit execution. A User would appear different depending on where and how it's instantiated, despite representing a row in the database.

I would instead implement something like below:

class User
  def course_role(course)
    memberships.find_by_course(course).role
  end
end

class Course
  def user_role(user)
    members.find_by_user(user).role
  end
end

This way, it's explicit that calling methods on either the User or Course to get the role depends on the respective memberships they are associated with.