refacto approving_instructor as a relation

app/models/authorization.rb

@@ -26,6 +26,16 @@ class Authorization < ApplicationRecord
     as: :entity,
     dependent: :nullify
 
+  has_one :approve_authorization_request_event,
+    -> { where(name: 'approve').order(created_at: :desc).limit(1) },
+    dependent: :nullify,
+    class_name: 'AuthorizationRequestEvent',
+    inverse_of: :entity
+
+  has_one :approving_instructor,
+    through: :approve_authorization_request_event,
+    source: :user
+
   scope :validated, -> { joins(:request).where(authorization_requests: { state: 'validated' }) }
 
   delegate :name, :kind, to: :request
@@ -68,14 +78,6 @@ def definition
     authorization_request_class.constantize.definition
   end
 
-  def approving_instructor
-    authorization_request_events
-      .where(name: 'approve')
-      .order(created_at: :desc)
-      .first
-      .try(:user)
-  end
-
   private
 
   def affect_snapshot_documents(request_as_validated)

spec/models/authorization_spec.rb

@@ -96,4 +96,30 @@
       end
     end
   end
+
+  describe '#approving_instructor' do
+    subject { authorization.approving_instructor }
+
+    let!(:authorization) { create(:authorization) }
+
+    context 'when there is no approving instructor' do
+      it { is_expected.to be_nil }
+    end
+
+    context 'when there is an approve event' do
+      let!(:event) { create(:authorization_request_event, entity: authorization, name: 'approve') }
+
+      it { is_expected.to eq(event.user) }
+    end
+
+    context 'when there is several approve events' do
+      let!(:events) do
+        create_list(:authorization_request_event, 3, entity: authorization, name: 'approve') do |event, index|
+          event.update(created_at: Time.zone.now - index.days)
+        end
+      end
+
+      it { is_expected.to eq(events.first.user) }
+    end
+  end
 end