[web2py] compute not working on update

[peter]

I am using web2py 1.99.7 

I have a compute as

db.image.thumb.compute=lambda r:THUMB(r['file'])


I update the whole database 

def update_images():
    rows=db(db.image).select()
    for row in rows:
        row.update(title=row.title)
    db.commit()


Yet, the computes have not occurred.

Similarly if I edit a record and submit, no compute. Yet if I add a record, 
the compute does take place.

Anyone any ideas why?

Thanks
Peter