I need to add Quantity field to journal items , the value of this field will be the same value of Done field of stock move , and if the journal entry created without stock move the value of Quantity field will be 1.

I created this code :

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

qty=fields.Float(string="QTY",default=1)
Which function can i use to make the value of qty equal to Done quantity in case of Journal entry created by stock move?