Add is_primary field to MAC address REST API serializer

netbox/dcim/api/serializers_/devices.py

@@ -290,11 +290,13 @@ class MACAddressSerializer(PrimaryModelSerializer):
         allow_null=True
     )
     assigned_object = GFKSerializerField(read_only=True)
+    is_primary = serializers.BooleanField(read_only=True)
 
     class Meta:
         model = MACAddress
         fields = [
             'id', 'url', 'display_url', 'display', 'mac_address', 'assigned_object_type', 'assigned_object_id',
-            'assigned_object', 'description', 'owner', 'comments', 'tags', 'custom_fields', 'created', 'last_updated',
+            'assigned_object', 'is_primary', 'description', 'owner', 'comments', 'tags', 'custom_fields', 'created',
+            'last_updated',
         ]
         brief_fields = ('id', 'url', 'display', 'mac_address', 'description')

netbox/dcim/tests/test_api.py

@@ -3880,3 +3880,25 @@ class MACAddressTestCase(APIViewTestCases.APIViewTestCase):
                 'mac_address': '00:00:00:00:00:06',
             },
         ]
+
+    def test_is_primary_field(self):
+        """
+        The read-only is_primary field should reflect whether the MAC address is the primary on its interface.
+        """
+        self.add_permissions('dcim.view_macaddress')
+
+        primary_mac = MACAddress.objects.get(mac_address='00:00:00:00:00:01')
+        non_primary_mac = MACAddress.objects.get(mac_address='00:00:00:00:00:02')
+
+        # Designate one MAC address as the primary on its interface
+        interface = primary_mac.assigned_object
+        interface.primary_mac_address = primary_mac
+        interface.save()
+
+        url = reverse('dcim-api:macaddress-detail', kwargs={'pk': primary_mac.pk})
+        response = self.client.get(url, **self.header)
+        self.assertTrue(response.data['is_primary'])
+
+        url = reverse('dcim-api:macaddress-detail', kwargs={'pk': non_primary_mac.pk})
+        response = self.client.get(url, **self.header)
+        self.assertFalse(response.data['is_primary'])