I made a field.char in purchase.order_form, and i can fill it:

from openerp.osv import fields, osv
class product_template(osv.Model):
    _name = 'purchase.order'
    _inherit = 'purchase.order'
    _columns = {
            'container_number': fields.char(string='Número contenedor', help='Número de contenedor', size = 30),

Now, i want to show that field as readonly in stock.view_picking_form.(in other module i created)  Now its showing the string, but now taking the data filled in puchase order.

from openerp.osv import fields, osv
class product_template(osv.Model):
    _name = 'stock.picking'
    _inherit = 'stock.picking'
    _columns = {
            'numero_containers': fields.related('origin','container_number',type="char",relation="purchase.order",string="Número de contenedor",readonly=True),
    }

What i am doing wrong.