count livestocks by type & set in herd model by signals

apps/livestock/apps.py

@@ -4,3 +4,6 @@ from django.apps import AppConfig
 class LivestockConfig(AppConfig):
     default_auto_field = 'django.db.models.BigAutoField'
     name = 'apps.livestock'
+
+    def ready(self):
+        import apps.livestock.signals

apps/livestock/signals.py

@@ -0,0 +1,31 @@
+from django.db.models.signals import post_save, post_delete
+from django.dispatch import receiver
+from django.db.models import Sum, Count, Q
+from .models import LiveStock
+
+
+def update_herd_statistics(instance: LiveStock):
+    """
+    update light & heavy livestock number in rancher herd
+    """
+    counts = LiveStock.objects.filter(
+        herd__rancher=instance.herd.rancher,
+        herd=instance.herd
+    ).aggregate(
+        light_livestock_number=Count('id', filter=Q(weight_type='L')),
+        heavy_livestock_number=Count('id', filter=Q(weight_type='H'))
+    )
+
+    instance.herd.light_livestock_number = counts['light_livestock_number']
+    instance.herd.heavy_livestock_number = counts['heavy_livestock_number']
+    instance.herd.save(update_fields=['light_livestock_number', 'heavy_livestock_number'])
+
+
+@receiver(post_save, sender=LiveStock)
+def update_herd_on_livestock_save(sender, instance, **kwargs):
+    update_herd_statistics(instance)
+
+
+@receiver(post_delete, sender=LiveStock)
+def update_herd_on_livestock_delete(sender, instance, **kwargs):
+    update_herd_statistics(instance)