Rails testing with RSpec double: ActiveRecord::AssociationTypeMismatch:

I am new to RSpec (not very experienced yet with RoR neither!)

I am working on a web app where I have a Course model. To create a new Course I need to use Teacher and Student objects.

I want to create a test on the Course model and for that I am trying to use instance_double to mock Teacher and Student. Here is my code

require 'rails_helper'

RSpec.describe Course, type: :model do
  it 'can be created' do
    # creating the teacher
    john = instance_double(Teacher)
    allow(john).to receive(:save).and_return(true)
    allow(john).to receive(:id).and_return(1)

    # creating the student
    francois = instance_double(Student)
    allow(francois).to receive(:save).and_return(true)
    allow(francois).to receive(:id).and_return(1)

    # creating the course
    testCourse = Course.new(teacher: john, student: francois, class_language: "EN")

    # testing the course
    expect(testCourse).to be_valid
  end
end

I get the next failure from RSpec

 1) Course can be created
     Failure/Error: testCourse = Course.new(teacher: john, student: francois, class_language: "EN")
     
     ActiveRecord::AssociationTypeMismatch:
       Teacher(#70253947615480) expected, got #<Double Teacher(id: integer, email: string, encrypted_password: string, first_name: string, last_name: string, about: text, reset_password_token: string, reset_password_sent_at: datetime, remember_created_at: datetime, created_at: datetime, updated_at: datetime)> which is an instance of RSpec::Mocks::Double(#70253946301600)
     # ./spec/models/course_spec.rb:16:in `block (2 levels) in <top (required)>'

Finished in 0.02549 seconds (files took 2.87 seconds to load)
1 example, 1 failure

My understanding is that ActiveRecord error is being caused by trying to associate a instance_double teacher and user with a course that expects a "real" teacher and student to be associated.

I have been trying many different ways, faking ID (see my code), or calling the models (see article here) themselves with

require 'student'
require 'teacher'

I saw this Stackoverflow post about similar issue, so I used non-deprecated syntax and it still doesn't work.

Any idea? Thanks!