Allow assignment of study groups for internal users

app/models/internal_user.rb

@@ -11,6 +11,8 @@ class InternalUser < User
   validates :password, confirmation: true, if: -> { password_void? && validate_password? }, on: :update, presence: true
   validate :password_strength, if: -> { password_void? && validate_password? }, on: :update
 
+  accepts_nested_attributes_for :study_group_memberships
+
   def activated?
     activation_state == 'active'
   end

app/models/study_group_membership.rb

@@ -1,9 +1,17 @@
 # frozen_string_literal: true
 
 class StudyGroupMembership < ApplicationRecord
+  ROLES = %w[learner teacher].freeze
+
   belongs_to :user, polymorphic: true
   belongs_to :study_group
 
+  before_save :destroy_if_empty_study_group_or_user
+
+  def destroy_if_empty_study_group_or_user
+    destroy if study_group.blank? || user.blank?
+  end
+
   enum role: {
     learner: 0,
     teacher: 1,