Restrict ticket selection for dependencies and parents

When adding a dependent or parent ticket, the choice field in the
ticket selection form excludes:

- All existing dependencies of the current ticket.
- All existing parent tickets of the current ticket.
- The current ticket itself.

The first two prevent immediate circular references: A dependency
cannot be a parent and vice versa.  Deeper circular references are
not covered by this: a ticket can still be its own grandchild.

They also prevent current behavior of throwing an `IntegrityException`
when selecting a dependency or parent.

The third one prevents also a quirky behavior: until now, specifying the
ticket itself as parent or dependency just does not save the
dependency and does not issue a warning either.

helpdesk/forms.py

@@ -591,6 +591,12 @@ class TicketDependencyForm(forms.ModelForm):
         model = TicketDependency
         exclude = ('ticket',)
 
+    def __init__(self, ticket, *args, **kwargs):
+        super(TicketDependencyForm,self).__init__(*args, **kwargs)
+
+        # Exclude duplicate tickets, myself, existing dependencies and parents
+        self.fields['depends_on'].queryset = Ticket.objects.exclude(status=Ticket.DUPLICATE_STATUS).exclude(id=ticket.id).exclude(depends_on__ticket=ticket).exclude(ticketdependency__depends_on=ticket)
+
 class TicketResolvesForm(forms.ModelForm):
     ''' Adds this ticket as a dependency for a different ticket '''
 
@@ -599,6 +605,12 @@ class TicketResolvesForm(forms.ModelForm):
         #exclude = ('depends_on',)
         fields = ('ticket',)
 
+    def __init__(self, ticket, *args, **kwargs):
+        super(TicketResolvesForm,self).__init__(*args, **kwargs)
+
+        # Exclude duplicate tickets, myself, existing dependencies and parents
+        self.fields['ticket'].queryset = Ticket.objects.exclude(status=Ticket.DUPLICATE_STATUS).exclude(id=ticket.id).exclude(depends_on__ticket=ticket).exclude(ticketdependency__depends_on=ticket)
+
 
 class MultipleTicketSelectForm(forms.Form):
     tickets = forms.ModelMultipleChoiceField(