How to add values from one model and insert the result into another using Django?

Question

I have this model that aims to register the available services and their respective values:

class Servico(models.Model):
    nome = models.CharField('Nome do serviço', max_length=255)
    preco = models.DecimalField('Preço do serviço', decimal_places=2)
    data_cadastro = models.DateTimeField('Data de cadastro',auto_now_add=True)
    
    def __str__(self):
        return self.nome

I created another model called sale, where you can register several services for a single sale. I would like to insert the sum of service prices within valor_total

class Venda(models.Model):
    FORMPAG_CHOICES = (
        ('CC', 'Cartão de Crédito'),
        ('CD', 'Cartão de Crédito'),
        ('D', 'Dinheiro'),
        ('T', 'Transferência (Entre contas/PIX)')
    )

    forma_pag = models.CharField('Forma de pagamento', max_length=2, choices=FORMPAG_CHOICES)
    cliente = models.ForeignKey(Cliente, verbose_name='Cliente', on_delete='models.PROTECT')
    servicos = models.ManyToManyField(Servico, verbose_name='Serviços prestados')
    data_cadastro = models.DateTimeField('Data da venda',auto_now_add=True)
    valor_total = # É aqui onde quero inserir o valor total da venda

Answer 1

Maybe create a Property in the Sell model forever calculate this for you. Something like below:

class Venda(models.Model):
  # ...

  @property
  def valor_total():
    servicos = self.servicos.all() # aplicar filtros padroes se precisar
    preco_total = 0.00
    for servico in servicos:
       preco_total += servico.preco
    return preco_total

the @property is practically to be able to call venda.valor_total and no need to call venda.valor_total()

I hope this helps.