I'm trying to work with association and session but have a bit of headhack.

First of all, I want to mentionned that the date sent by the server are :
- with nested data
- without ID of the parent in associated data

Please find the fiddle : https://fiddle.sencha.com/#fiddle/f5i

What I would like to achieve is that when loading the associated data, it refer to previously loaded data (here I have a list of article, a list of events and articles has related events, I want that nested data in the article refer to the same model as in the event store.)
I understood session is made in this purpose.

But with my code here... I'm not able to go to the first step, I get a error :
PHP Code:
TypeErrorfield is null

If someone can provide a little bit of help/guidance.

Thanks in advance,