Prevent joining the same class again

server/users/mutations.py

@@ -1,11 +1,13 @@
 import graphene
 from django.contrib.auth import update_session_auth_hash
 from django.core.exceptions import PermissionDenied
+from django.db.models import Q
 from graphene import relay
 
 from api.utils import get_object
 from users.inputs import PasswordUpdateInput
 from users.models import SchoolClass, UserSetting
+from users.schema import SchoolClassNode
 from users.serializers import PasswordSerialzer, AvatarUrlSerializer
 
 
@@ -111,17 +113,18 @@ class JoinClass(relay.ClientIDMutation):
         code = graphene.String(required=True)
 
     success = graphene.Boolean()
+    school_class = graphene.Field(SchoolClassNode)
 
     @classmethod
     def mutate_and_get_payload(cls, root, info, **kwargs):
         user = info.context.user
         code = kwargs.get('code')
         try:
-            school_class = SchoolClass.objects.get(code__iexact=code)
+            school_class = SchoolClass.objects.get(Q(code__iexact=code) & ~Q(users=user))
 
             school_class.users.add(user)
 
-            return cls(success=True)
+            return cls(success=True, school_class=school_class)
         except SchoolClass.DoesNotExist:
             raise CodeNotFoundException('Code ist nicht gültig')