diff -r 205804d9f142 -r 91c44b3fd11f src/hdalab/models/dataviz.py
--- a/src/hdalab/models/dataviz.py	Thu Nov 13 16:07:22 2014 +0100
+++ b/src/hdalab/models/dataviz.py	Fri Nov 14 03:07:20 2014 +0100
@@ -4,8 +4,10 @@
 
 @author: ymh
 '''
+from hdabo.models import Tag, Datasheet
+from hdalab import fields
+
 from django.db import models
-from hdabo.models import Tag, Datasheet
 
 
 class TagYears(models.Model):
@@ -44,7 +46,7 @@
 class DbpediaFields(models.Model):
 
     dbpedia_uri = models.URLField(max_length=2048, blank=False, null=False, db_index=True, unique=False)
-    tag = models.OneToOneField(Tag, blank=True, null=True, db_index=True, related_name="dbpedia_fields")
+    tag = fields.OneToOneField(Tag, blank=True, null=True, db_index=True, related_name="dbpedia_fields", related_default=lambda instance: None)
     abstract = models.TextField(blank=True, null=True)
     thumbnail = models.URLField(max_length=2048, blank=True, null=True, db_index=False)    
     label = models.CharField(max_length=2048, unique=False, blank=True, null=True)