add a check if a slug of Fachschaftssitzung exists already

fet2020/posts/models.py

@@ -1,4 +1,5 @@
 from django.contrib.auth.models import User
+from django.core.validators import ValidationError
 from django.db import models
 from django.db.models import Q
 from django.urls import reverse
@@ -168,7 +169,8 @@ class Post(models.Model):
         # TODO: Explain why this image is selected on save of the image
         # Query all posts that have a slug that equals one of the tags
         posts1 = Post.objects.filter(
-            slug__in=self.tags.names()).filter(image__isnull=False)[0:1].all()
+            slug__in=self.tags.names()
+        ).filter(image__isnull=False)[0:1].all()
         if len(posts1) > 0:
             return posts1.get().image
 
@@ -186,11 +188,12 @@ class Post(models.Model):
         "save the post with some defaults"
         if (self.id is None) and (not self.slug):
             self.slug = slugify(self.public_date.date()) + "-" + slugify(self.title)
-
         super().save(*args, **kwargs)
 
-        self.tags.set(*re.findall(r'\#([\d\w-]+)', str(self.subtitle)),
-                      *re.findall(r'\#([\d\w-]+)', str(self.title)))
+        self.tags.set(
+            *re.findall(r'\#([\d\w-]+)', str(self.subtitle)),
+            *re.findall(r'\#([\d\w-]+)', str(self.title))
+        )
 
     def __str__(self):
         return "Post (%s, %s): %s " % (self.slug, self.public_date.strftime("%d.%m.%Y"), self.title)
@@ -262,6 +265,12 @@ class FetMeeting(Event):
         verbose_name = "Fet Sitzung"
         verbose_name_plural = "Fet Sitzungen"
 
+    def clean(self):
+        slug = slugify(self.event_start.date()) + "-" + slugify("Fachschaftssitzung")
+
+        if Post.objects.filter(slug=slug).count() != 0:
+            raise ValidationError(_('Es existiert bereits eine Sitzung mit demselben Datum.'))
+
     def save(self, *args, **kwargs):
         self.title = "Fachschaftssitzung"
         self.slug = slugify(self.event_start.date()) + "-" + slugify(self.title)