Odoo SaaS v9

I would like to get the same value of  Issued_total from res.partner for a custom computed field.

I notice that the python code for the Issued_total is 

@api.multi def _compute_issued_total(self): """ Returns the issued total as will be displayed on partner view """ today = fields.Date.context_today(self) for partner in self: domain = partner.get_followup_lines_domain(today, overdue_only=True) issued_total = 0 for aml in self.env['account.move.line'].search(domain): issued_total += aml.amount_residual partner.issued_total = issued_total

How would I make a custom computed field do the equivalent of that python code does? So that I would have the same value as Issued_total would for my custom field?

Please note: I need it to be Odoo SaaS friendly.