[Grok-dev] unique item in container question

[modiadi]

Hello,

I have a grok.AddForm wich adds a NewsItem in a NewsContainer. I try  
to write some kind of validation so I dont have duplicate news in the  
container.

As I see I have 3 a few options:

1. Write an invariant in the NewsItem model where I check if the news  
is unique in the *container*. I don't see this as a good option  
because I have to explicitly refer to the container in side a model.

2. Write the validation in the Add grok.action. The idea is that just  
before adding the new item in the container I check if I have  
duplicates. This was easy to do.

My problem is that I can't(or I don't know how to) return an error to  
the user from this place.

     @grok.action('Add event')
       def add(self,**data):
           ...... do some validation
	  return None?

Do you have any suggestions ?

Thank you !