Django admin model calc something before save to db

#models.py

'''orderMain'''

class OrderMain(models.Model):

    orderId = models.UUIDField(primary_key=True, max_length=30)#sum(OrderDetail.amount)

    sumAmount = models.DecimalField(max_digits=13,decimal_places=3,null=True) #sum(orderDetail.amount)

    class Meta:

        db_table = 'orderMain'

 

    def __str__(self):

        return self.orderId

    def save(self, *args, **kwargs):

        super(OrderMain, self).save(*args, **kwargs)

'''orderDetail'''

class OrderDetail(models.Model):

    orderId = models.ForeignKey(OrderMain,to_field='orderId',db_column='orderId',on_delete=models.CASCADE)

    amount = models.DecimalField(max_digits=8,decimal_places=2)

    class Meta:

        db_table = 'orderDetail'

    def __str__(self):

        return ''

#admin.py

class OrderDetailInLine(admin.TabularInline):

    model = OrderDetail

    extra = 1

@admin.register(OrderMain)

class OrderMainAdmin(admin.ModelAdmin):

    inlines = [OrderDetailInLine,]

    list_display = ['orderId','sumAmount']

    readonly_fields = ['orderId','sumAmount']

    fields = ('orderId', 'sumAmount')

#Questions: Can I calc orderMain.sumAmount = sum(orderDetail.amount)  before objects save to db?

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