core: prevent duplicate fixed_username/email

passbook/core/forms/invitations.py

@@ -1,13 +1,29 @@
 """passbook core invitation form"""
 
 from django import forms
+from django.core.exceptions import ValidationError
+from django.utils.translation import gettext as _
 
-from passbook.core.models import Invitation
+from passbook.core.models import Invitation, User
 
 
 class InvitationForm(forms.ModelForm):
     """InvitationForm"""
 
+    def clean_fixed_username(self):
+        """Check if username is already used"""
+        username = self.cleaned_data.get('fixed_username')
+        if User.objects.filter(username=username).exists():
+            raise ValidationError(_('Username is already in use.'))
+        return username
+
+    def clean_fixed_email(self):
+        """Check if email is already used"""
+        email = self.cleaned_data.get('fixed_email')
+        if User.objects.filter(email=email).exists():
+            raise ValidationError(_('E-Mail is already in use.'))
+        return email
+
     class Meta:
 
         model = Invitation