Rails Devise STI - undefined local variable or method `users' for <QuizSession>

Question

I'm using single table inheritance so my models Student and Teacher inherit from the same Devise model User (attributes are the same, only the relationships to other models are different).

Now I was trying to display data from an instance of the model QuizSession, which has a one to one relationship with Teacher and a one to many relationship with Student, but I keep getting the error: undefined local variable or method 'users' for #< QuizSession:0xb48e740 > .

Here are my models:

# app/models/user.rb:
class User < ApplicationRecord
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
  belongs_to :quiz_session, optional: true

  # Which users subclass the User model
  def self.types
    %w(Teacher Student)
  end

  # Add scopes to the parent models for each child model
  scope :teachers, -> { where(type: 'Teacher') }
  scope :students, -> { where(type: 'Student') }
end

# app/models/teacher.rb:
class Teacher < User
end

# app/models/student.rb:
class Student < User
end

# app/models/quiz_session.rb:
class QuizSession < ApplicationRecord
  belongs_to :quiz
  has_one :teacher
  has_many :students

  delegate :teachers, :students, to: :users #<-- this is apparently where the error occurs
end

EDIT: The problem seems to occur when I try to call @quiz_session.students. While the correct QuizSession record is found, apparently it can't resolve .students? I don't understand why, because the User model does have an attribute quiz_session_id which the student model should inherit.

Answer 1

Change this line:

delegate :teachers, :students, to: :users

To this:

delegate :teachers, :students, to: :user

The reason for this change is because your model is User, not Users.