feat: sla-7 ensure applied_sla uniqueness (#8938)

* feat: refactor fetching sla in action service

* chore: modify spec

* chore: ensure applied_sla uniqueness

* chore: review fixes

* feat: add unique index on applied_sla

* chore: fix spec

* chore: add new specs to improve coverage

* chore: improve spec

* Update spec/enterprise/services/enterprise/action_service_spec.rb

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

spec/enterprise/services/enterprise/action_service_spec.rb

@@ -8,16 +8,42 @@ describe ActionService do
     let(:conversation) { create(:conversation, account: account) }
     let(:action_service) { described_class.new(conversation) }
 
-    it 'adds the sla policy to the conversation and create applied_sla entry' do
-      action_service.add_sla(sla_policy)
-      expect(conversation.reload.sla_policy_id).to eq(sla_policy.id)
+    context 'when sla_policy_id is present' do
+      it 'adds the sla policy to the conversation and create applied_sla entry' do
+        action_service.add_sla([sla_policy.id])
+        expect(conversation.reload.sla_policy_id).to eq(sla_policy.id)
 
-      # check if appliedsla table entry is created with matching attributes
-      applied_sla = AppliedSla.last
-      expect(applied_sla.account_id).to eq(account.id)
-      expect(applied_sla.sla_policy_id).to eq(sla_policy.id)
-      expect(applied_sla.conversation_id).to eq(conversation.id)
-      expect(applied_sla.sla_status).to eq('active')
+        # check if appliedsla table entry is created with matching attributes
+        applied_sla = AppliedSla.last
+        expect(applied_sla.account_id).to eq(account.id)
+        expect(applied_sla.sla_policy_id).to eq(sla_policy.id)
+        expect(applied_sla.conversation_id).to eq(conversation.id)
+        expect(applied_sla.sla_status).to eq('active')
+      end
+    end
+
+    context 'when sla_policy_id is not present' do
+      it 'does not add the sla policy to the conversation' do
+        action_service.add_sla(nil)
+        expect(conversation.reload.sla_policy_id).to be_nil
+      end
+    end
+
+    context 'when conversation already has a sla policy' do
+      it 'does not add the new sla policy to the conversation' do
+        existing_sla_policy = sla_policy
+        new_sla_policy = create(:sla_policy, account: account)
+        conversation.update!(sla_policy_id: existing_sla_policy.id)
+        action_service.add_sla([new_sla_policy.id])
+        expect(conversation.reload.sla_policy_id).to eq(existing_sla_policy.id)
+      end
+    end
+
+    context 'when sla_policy is not found' do
+      it 'does not add the sla policy to the conversation' do
+        action_service.add_sla([sla_policy.id + 1])
+        expect(conversation.reload.sla_policy_id).to be_nil
+      end
     end
   end
 end