Hi Barlet
Sorry, your understanding of Edwin's solution is incorrect.
Also, apologies, I missed one important fact that Edwin's attribute was a Virtual Attribute and Yours is a DB Attribute.
Lets move a step back.
Can you please create a new Attribute in ZKNA1 as CUST_GRP (*remove the old one) and make it a virtual attribute?
In the Program, select from KNVV where KUNNR = OBJECT-KEY (I am not sure how you will fetch the other key fields).
Then assign to this property.
See if this works.
regards,
Modak