How can I join two model in django - django

Question

I want to join two model. I am using postgresql. But the problem is I am not using any foreign key or many to field. But I am using same unique field for both table. So I want depends on thats field I can query from both table. "invoice" is the field. My models are
class Salesinvoice(models.Model):
id = models.AutoField(primary_key=True)
date = models.DateField(default=date.today)
invoice = models.CharField(max_length=20)
customer = models.CharField(max_length=100)
product_name = models.CharField(max_length=80)
price = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
quantity = models.IntegerField(default=0)
class Salespayment(models.Model):
id = models.AutoField(primary_key=True)
date = models.DateField(default=date.today)
invoice = models.CharField(max_length=20)
amount = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
discount = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
payment = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
What should be my Views for join above both table depends on invoice field.

Answer 1

Change your models, specifically:
SalesInvoice.invoice to be unique. If you cannot guarantee that this value will be unique - think again about your models relationships and the data.
make SalesPayment.invoice a ForeignKey field to SalesInvoice. Read about what on_delete=PROTECT does and what other options there are: https://docs.djangoproject.com/en/4.0/ref/models/fields/#foreignkey
you don't need to explicitly add id fields. This is done by Django anyway.
class SalesInvoice(models.Model):
date = models.DateField(default=date.today)
invoice = models.CharField(max_length=20, unique=True) # changed
customer = models.CharField(max_length=100)
product_name = models.CharField(max_length=80)
price = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
quantity = models.IntegerField(default=0)
class SalesPayment(models.Model):
date = models.DateField(default=date.today)
invoice = models.ForeignKey(SalesInvoice, on_delete=PROTECT) # changed
amount = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
discount = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
payment = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
Django will automatically support the following lookups and much more:
# get all payments for an invoice (could be empty)
payments = salesInvoice.salespayment_set.all() # this is a queryset
# get the invoice of a specific payment by date of a customer
payment = SalesPayment.objects.filter(customer=cust_pk, date=some_date).first()
if payment:
invoice = payment.invoice.invoice # this is your identifying string

Answer 2

For example, add a function to Salesinvoice class:
class Salesinvoice(models.Model):
id = models.AutoField(primary_key=True)
....
quantity = models.IntegerField(default=0)
def payment(self):
if Salespayment.objects.filter(invoice = self.invoice).count() == 1:
return Salespayment.objects.get(invoice = self.invoice)
else:
return None
So, when you need the related Salespayment in your views.py , just do:
salesinvoice = Salesinvoice.objects.get(...)
salespayment = salesinvoice.payment()
Same in templates:
{{salesinvoice.payment.anyfield}}