Here is what I did:

class pos_order_line(osv.osv):
    _inherit = "pos.order.line"
    _columns = {
        'order_line_state_id': fields.many2one('pos.order.line.state', "Order Line State"),
    }

class pos_order_line_state(osv.osv):
    _name = "pos.order.line.state"

    _columns = {
        'name': fields.char('Name', size=18),
        'sequence': fields.integer('Sequence'),
    }

And in javascript I do:

var order = this.pos.get('selectedOrder');    
var line = order.getSelectedLine();

But line doesn't have order_line_state_id even though the column is in the database (checked with PGadminIII)

What am I doing wrong??