I'm trying to apply a global discount to invoices (v13). I don't want to modify the way Odoo calculates the invoice amount, so all I'm doing is change the discount field for every account.move.line of the invoice.

I create the global_discount field in account.move:​

class AccountMove(models.Model):
    _inherit = 'account.move'

    global_discount = fields.Float(string='Global Discount (%)', digits='Discount', default=0.0)
​

Now, I want to change the discount field when global_discount is modified, but @api.onchange is not being called when doing so. It doesn't work adding either the parent reference or a related field to the decorator:

class AccountMoveLine(models.Model):
    _inherit = 'account.move.line'

    global_discount = fields.Float(related='move_id.global_discount')

    # This method is not being called when changing global_discount
    @api.onchange('move_id.global_discount', 'global_discount')
    def _compute_discount(self):
        for line in self:
            # Simplified for easy reading, I'm not actually doing this
            line.discount = line.global_discount

Even if I make discount a computed field and use @api.depends, its onchange methods won't be called.
If I modify the lines from an onchange method in the parent, the discount onchange methods won't be called and  price_subtotal will not change:

class AccountMove(models.Model):
    _inherit = 'account.move'

    @api.onchange('global_discount')
    def _compute_discount(self):
        for invoice in self:
            for line in invoice.invoice_line_ids:
              line._compute_discount()

I have also tried calling the onchange methods manually, but I'm definitely doing something wrong because nothing new happens:

@api.onchange('global_discount')
    def _compute_discount(self):
        for invoice in self:
            for line in invoice.invoice_line_ids:
                line._compute_discount()
                spec = line._onchange_spec()
                values = dict(line._cache)

                # onchange returns and empty dict {}
                updates = line.onchange(values, 'discount', spec)

                value = updates.get('value', {})

                for name, val in value.items():
                    if isinstance(val, tuple):
                        value[name] = val[0]

                values.update(value)
                line.update(values)

I'm missing something on how Odoo calls onchange methods from python, but I can't figure it out.