adds auditing for school class (#460)

Closes
https://github.com/RaspberryPiFoundation/digital-editor-issues/issues/340

* Adds auditing for School Class

Author: conorriches

app/models/school_class.rb

@@ -9,6 +9,12 @@ class SchoolClass < ApplicationRecord
   validates :name, presence: true
   validate :teacher_has_the_school_teacher_role_for_the_school
 
+  has_paper_trail(
+    meta: {
+      meta_school_id: ->(cm) { cm.school&.id }
+    }
+  )
+
   def self.teachers
     User.from_userinfo(ids: pluck(:teacher_id))
   end

spec/models/school_class_spec.rb

@@ -2,7 +2,7 @@
 
 require 'rails_helper'
 
-RSpec.describe SchoolClass do
+RSpec.describe SchoolClass, versioning: true do
   before do
     stub_user_info_api_for(teacher)
   end
@@ -150,4 +150,12 @@
       expect(pair).to eq([school_class, nil])
     end
   end
+
+  describe 'auditing' do
+    subject(:school_class) { create(:school_class, teacher_id: teacher.id, school:) }
+
+    it 'enables auditing' do
+      expect(school_class.versions.length).to(eq(1))
+    end
+  end
 end