diff -r d162cb0c4386 -r 46ad324f6fe4 src/hdalab/models/dataviz.py
--- a/src/hdalab/models/dataviz.py	Fri Nov 14 17:06:02 2014 +0100
+++ b/src/hdalab/models/dataviz.py	Sun Nov 16 18:09:42 2014 +0100
@@ -12,7 +12,7 @@
 
 class TagYears(models.Model):
     
-    tag = models.ForeignKey(Tag, related_name="years", blank=False, null=False, db_index=True)
+    tag = models.ForeignKey(Tag, related_name="years", blank=False, null=False, db_index=True, unique=True)
     start_year = models.IntegerField(blank=False, null=False, db_index=True)
     end_year = models.IntegerField(blank=False, null=False, db_index=True)
     
@@ -46,7 +46,7 @@
 class DbpediaFields(models.Model):
 
     dbpedia_uri = models.URLField(max_length=2048, blank=False, null=False, db_index=True, unique=False)
-    tag = fields.OneToOneField(Tag, blank=True, null=True, db_index=True, related_name="dbpedia_fields", related_default=lambda instance: None)
+    tag = fields.OneToOneField(Tag, 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)