inheritance - sort issue

[Ferdinand]

sort of inherited fields raise error

in my case (custom program) koo reports an error if I want to sort a column which does not belong th the underlying postgres table.

[25]: ProgrammingError: column "date_planning" does not exist
[26]: LINE 1: ...les.id from "chricar_budget_lines_sales" order by date_plann...

class chricar_budget_lines_sales(osv.osv):
     _name = "chricar.budget_lines_sales"
     _table = "chricar_budget_lines_sales"
     _inherits = {'c2c_budget.line': 'budget_line_id'}

 "date_planning" is defined in 'c2c_budget.line'