this.pos
is a reference to the original unmodified PosModel, not the one you have inherited. That's why you have the order_line_state_id
in the database but it's not a member variable of the JS object.
I don't yet have any experience on JavaScript hacking in OpenERP so I don't know how to solve this. You might want to browse the JS code of the point_of_sale
module and make the PosModel use your custom module instead of the original one.