How can I show the pricelist discount in a sale order line and make it editable?

Question

I would like to show the discount of the pricelist. It matches with the price_discount field of the model product.pricelist.item.

But once I get the discount from that model I want to make it editable. How can I achieve this?

I tried this code with no luck. I make the field discount_percentage invisible, I only use it for recomputing the field.

discount_percentage = fields.Float(
    compute='_compute_discount_percentage',
    string=_('Discount (%)'),
    digits=dp.get_precision('Discount'),
)

discount_manual = fields.Float(
    string=_('Discount dummy (%)'),
    digits=dp.get_precision('Discount'),
)

@api.one
@api.depends('product_id', 'product_id.lst_price', 'price_reduce', 'price_unit')
def _compute_discount_percentage(self):
    _logger.debug('-- COMPUTE DISCOUNT PERCENTAGE --')
    if not self.discount_manual:
        if self.product_id.lst_price:
            disc = (self.product_id.lst_price - self.price_reduce) * 100 / self.product_id.lst_price
        else:
            disc = 0            
        self.discount_percentage = disc
        self.discount_manual = self.discount_percentage
    else:
        self.discount_percentage = self.discount_manual

Is there a function where I can get the discount using the product_id and pricelist_id?

Should I use a SQL query in the product_id_change method in order to get the discount?