It's not fair to use related field here. Related field will write back the value to the currency table.

If you only need to see the value, you could use a functional field.

@api.one
@api.depends('currency_id', 'currency_id.rate')
def _compute_amount(self):
    self.currency_rate = self.currency_id.rate
currency_rate = fields.Float('Currency Rate', compute='_currency_rate', store=True)