Allow internal user to be part of an study group

app/controllers/concerns/lti.rb

@@ -202,7 +202,7 @@ module Lti
             else
               StudyGroup.find_or_create_by(external_id: @provider.resource_link_id, consumer: @consumer)
             end
-    group.users << @current_user unless group.users.include? @current_user
+    group.external_users << @current_user unless group.external_users.include? @current_user
     group.save
     session[:study_group_id] = group.id
   end

app/controllers/study_groups_controller.rb

@@ -10,11 +10,11 @@ class StudyGroupsController < ApplicationController
   end
 
   def show
-    @search = @study_group.users.ransack(params[:q])
+    @search = @study_group.external_users.ransack(params[:q])
   end
 
   def edit
-    @search = @study_group.users.ransack(params[:q])
+    @search = @study_group.external_users.ransack(params[:q])
     @members = StudyGroupMembership.where(user: @search.result, study_group: @study_group)
   end
 

app/models/study_group.rb

@@ -2,13 +2,15 @@
 
 class StudyGroup < ApplicationRecord
   has_many :study_group_memberships, dependent: :destroy
-  # Use `ExternalUser` as `source_type` for now.
-  # Using `User` will lead ActiveRecord to access the inexistent table `users`.
-  # Issue created: https://github.com/rails/rails/issues/34531
-  has_many :users, through: :study_group_memberships, source_type: 'ExternalUser'
+  has_many :external_users, through: :study_group_memberships, source_type: 'ExternalUser', source: :user
+  has_many :internal_users, through: :study_group_memberships, source_type: 'InternalUser', source: :user
   has_many :submissions, dependent: :nullify
   belongs_to :consumer
 
+  def users
+    external_users + internal_users
+  end
+
   def to_s
     if name.blank?
       "StudyGroup " + id.to_s