fix changing refs to compounds creating duplicates in other tables

such tables being CompoundTanimoto and DrugbankCompoundTanimoto
WIP on #212

ippisite/ippidb/models.py

@@ -951,12 +951,10 @@ class Compound(AutoFillableModel):
         for cpr in CompoundPKResult.objects.filter(compound=self):
             cpr.compound = replacing_compound
             cpr.save()
-        for ct in CompoundTanimoto.objects.filter(compound=self):
-            ct.compound = replacing_compound
-            ct.save()
-        for dct in DrugbankCompoundTanimoto.objects.filter(compound=self):
-            dct.compound = replacing_compound
-            dct.save()
+        # remove similarities with current component, in case any had been
+        # stored
+        CompoundTanimoto.objects.filter(compound=self).delete()
+        DrugbankCompoundTanimoto.objects.filter(compound=self).delete()
 
 
 class CompoundTanimoto(models.Model):