Entering calculated data in a Qgis vector layer: how?

[Jenske]

I have a vector layer (shape-file), created in Qgis and containing (amongst others) a field "len_km", that should contain a calculated value -- in this case the length of the line in kilometer --, but ... there seems to be _no_ way to update a table in Qgis with this kind of calculated data.

I am right, saying that I need to create PostgreSQL-layers in order to have this option? Or is it possible to do this using GRASS?

Or short: how can I let Qgis enter calculated values in fields of maps?

[GuZzO]

i think this feature is planned for future release of QGis.

The other way is to use postgis as you suggest