works better now

markt/admin.py

@@ -9,7 +9,7 @@ class OrderAdmin(admin.ModelAdmin):
 
 @admin.register(Factuur)
 class FactuurAdmin(admin.ModelAdmin):
-    list_display = ("Factuur_ID", "Klant_ID", "total" )
+    list_display = ("Factuur_ID", "Klant_ID", "Total" )
     
 
 

markt/migrations/0011_auto_20200524_1559.py

@@ -0,0 +1,21 @@
+# Generated by Django 3.0.6 on 2020-05-24 18:59
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('markt', '0010_auto_20200524_1530'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='factuur',
+            name='Subtotal',
+        ),
+        migrations.RemoveField(
+            model_name='factuur',
+            name='total',
+        ),
+    ]

markt/models.py

@@ -1,6 +1,7 @@
 from django.db import models
 # Used to generate URLs by reversing the URL patterns
 from django.urls import reverse
+from django.db.models import Sum
 
 # Create your models here.
 
@@ -135,7 +136,6 @@ class Order(models.Model):
         new_amount = self.Voorraad_ID.Voorraad_Amount - self.Amount
         if new_amount < 0:
             raise ValueError("Er zijn maar {} in voorraad".format(self.Voorraad_ID.Voorraad_Amount))
-
         self.Voorraad_ID.Voorraad_Amount -= self.Amount
         self.Voorraad_ID.save()
         return super(Order, self).save(*args, **kwargs)
@@ -146,8 +146,8 @@ class Factuur(models.Model):
     Klant_ID = models.ForeignKey("Klant", on_delete=models.PROTECT)
     Orders = models.ManyToManyField("Order", null=True, blank=True)
     Korting_percent = models.FloatField()
-    Subtotal = models.FloatField(null=True, blank=True)
-    total = models.FloatField(null=True, blank=True)
+    # Subtotal = models.FloatField(null=True, blank=True)
+    # total = models.FloatField(null=True, blank=True)
 
     def get_absolute_url(self):
         """Returns the url to access a particular instance of the model."""
@@ -157,12 +157,26 @@ class Factuur(models.Model):
         """String for representing the Model object."""
         return str(self.Factuur_ID)
 
-    def save(self, *args, **kwargs):
-        super(Factuur, self).save(*args, **kwargs)
-        self.Subtotal = sum([i.Prijs for i in self.Orders.all()])
-        self.total = self.Subtotal * (1 - self.Korting_percent/100)
+    @property
+    def Subtotal(self):
+        if self.pk:
+            return self.Orders.all().aggregate(Sum('Prijs'))['Prijs__sum']
+        else:
+            return 0
 
-        return super(Factuur, self).save(*args, **kwargs)
+    @property
+    def Total(self):
+        if self.pk:
+            return (100 - self.Korting_percent) * self.Subtotal / 100
+        else:
+            return 0
+
+    # def save(self, *args, **kwargs):
+    #     super(Factuur, self).save(*args, **kwargs)
+    #     self.Subtotal = sum([i.Prijs for i in self.Orders.all()])
+    #     self.total = self.Subtotal * (1 - self.Korting_percent/100)
+
+    #     return super(Factuur, self).save(*args, **kwargs)
 
     class Meta:
         db_table = "Factuur"