No module is needed, just put inherit the _make_po_get_domain function and put the origin field in the domain.

soon it starts to create purchase orders linked to the sales order.

def _make_po_get_domain(self, partner):

gpo = self.rule_id.group_propagation_option

group = (gpo == 'fixed' and self.rule_id.group_id) or \

(gpo == 'propagate' and self.group_id) or False

# import ipdb; ipdb.set_trace()

domain = (

('partner_id', '=', partner.id),

('state', '=', 'draft'),

('origin', '=', self.origin),

('picking_type_id', '=', self.rule_id.picking_type_id.id),

('company_id', '=', self.company_id.id),

('dest_address_id', '=', self.partner_dest_id.id))

if group:

domain += (('group_id', '=', group.id),)

return domain