Don't process payload_attribute_mapping when it's blank

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/321101

ee/app/graphql/ee/mutations/alert_management/http_integration/http_integration_base.rb

@@ -10,12 +10,6 @@ module EE
 
           private
 
-          def payload_attribute_mapping(mappings)
-            Array(mappings).each_with_object({}) do |m, h|
-              h[m.field_name] = { path: m.path, type: m.type, label: m.label }
-            end
-          end
-
           def validate_payload_example!(payload_example)
             return if ::Gitlab::Utils::DeepSize.new(payload_example).valid?
 
@@ -30,9 +24,23 @@ module EE
 
             validate_payload_example!(args[:payload_example])
 
-            args.slice(*base_args.keys, :payload_example).merge(
-              payload_attribute_mapping: payload_attribute_mapping(args[:payload_attribute_mappings])
-            )
+            args
+              .slice(*base_args.keys, :payload_example)
+              .merge(payload_attribute_mapping_params(args))
+          end
+
+          def payload_attribute_mapping_params(args)
+            # Don't process payload_attribute_mapping when it's not part of a mutation params.
+            # Otherwise, it's going to reset already persisted value.
+            return {} unless args.key?(:payload_attribute_mappings)
+
+            { payload_attribute_mapping: payload_attribute_mapping(args[:payload_attribute_mappings]) }
+          end
+
+          def payload_attribute_mapping(mappings)
+            Array(mappings).each_with_object({}) do |m, h|
+              h[m.field_name] = { path: m.path, type: m.type, label: m.label }
+            end
           end
         end
       end

ee/spec/requests/api/graphql/mutations/alert_management/http_integration/update_spec.rb

@@ -67,6 +67,54 @@ RSpec.describe 'Updating an existing HTTP Integration' do
   it_behaves_like 'validating the payload_example'
   it_behaves_like 'validating the payload_attribute_mappings'
 
+  context 'when the custom mappings attributes are not part of the mutation variables' do
+    let_it_be(:payload_example) do
+      { 'alert' => { 'name' => 'Test alert', 'desc' => 'Description' } }
+    end
+
+    let_it_be(:mapping) do
+      {
+        'title' => { 'path' => %w(alert name), 'type' => 'string', 'label' => 'Title' },
+        'description' => { 'path' => %w(alert desc), 'type' => 'string' }
+      }
+    end
+
+    let_it_be(:integration) do
+      create(:alert_management_http_integration, project: project, payload_example: payload_example, payload_attribute_mapping: mapping)
+    end
+
+    let(:mutation) do
+      variables = {
+        id: GitlabSchema.id_from_object(integration).to_s,
+        name: 'Modified Name'
+      }
+      graphql_mutation(:http_integration_update, variables) do
+        <<~QL
+          clientMutationId
+          errors
+          integration {
+            id
+            name
+          }
+        QL
+      end
+    end
+
+    it 'does not resets the custom mapping attributes', :aggregate_failures do
+      post_graphql_mutation(mutation, current_user: current_user)
+
+      integration_response = mutation_response['integration']
+
+      expect(response).to have_gitlab_http_status(:success)
+      expect(integration_response['id']).to eq(GitlabSchema.id_from_object(integration).to_s)
+      expect(integration_response['name']).to eq('Modified Name')
+
+      integration.reload
+      expect(integration.payload_example).to eq(payload_example)
+      expect(integration.payload_attribute_mapping).to eq(mapping)
+    end
+  end
+
   context 'with the custom mappings feature unavailable' do
     before do
       stub_licensed_features(multiple_alert_http_integrations: false)