Allow entrances to be edited with the correct parent url. Commit changes to caves when adding a new entrance. Order entrances alphabetically

core/forms.py

@@ -116,11 +116,21 @@ class EntranceForm(ModelForm):
 CaveAndEntranceFormSet = modelformset_factory(CaveAndEntrance, exclude=('cave',))
 
 class EntranceLetterForm(ModelForm):
-    '''Can't see what this does at all. called twice from views.caves
+    '''Form to link entrances to caves, along with an entrance number.
+    
+    Nb. The relationship between caves and entrances has historically been a many to many relationship.  
+    With entrances gaining new caves and letters when caves are joined.
     '''
     class Meta:
         model = CaveAndEntrance
         exclude = ('cave', 'entrance')
+    
+    def full_clean(self):
+        super(EntranceLetterForm, self).full_clean()
+        try:
+            self.instance.validate_unique()
+        except forms.ValidationError as e:
+            self._update_errors(e)