You need to inherit the read_group
method of the model/class:
For example your field name is invoice_due_local_currency
class your_class(models.Model):
# ...
@api.model
def read_group(self, domain, fields, groupby, offset=0, limit=None, orderby=False, lazy=True):
res = super(your_class, self).read_group(domain, fields, groupby, offset=offset, limit=limit, orderby=orderby, lazy=lazy)
if 'invoice_due_local_currency' in fields:
for line in res:
if '__domain' in line:
lines = self.search(line['__domain'])
total_invoice_due = 0.0
for record in lines:
total_invoice_due += record.invoice_due_local_currency
line['invoice_due_local_currency'] = total_invoice_due
return res