Reverse One-to-Many-Relationship in Django - django-models

Question

My app links invoices, contracts and services with Many-to-One-Relationships:
class Invoice(models.Model):
contract = models.ForeignKey(Contract, on_delete=models.CASCADE)
class Contract(models.Model):
service = models.ForeignKey(Service, on_delete=models.CASCADE)
Whenever a new invoice is registered, it can be linked to a service and split/billed internally. Unfortunately, some contracts/invoices need to be linked to more than one service according to a fixed split (e.g. 30/70).
For this to work on the surface, I could to reverse the relationship between contracts and services –
class Service(models.Model):
contract = models.ForeignKey(Contract, on_delete=models.CASCADE)
– or change the ForeignKey field on the Contract class to a ManyToManyField.
But in both cases, I will not be able to get back from the invoice to the service easily anymore, as with the following statement:
invoices = Invoice.objects.filter(models.Q(contract__service__building=self.tenant.unit.building), models.Q(begin__lte=self.begin, end__gt=self.begin) | models.Q(begin__gt=self.begin, begin__lt=self.end))
Is it wise to insert an intermediate helper model (ContractService) with two ForeignKey fields to keep the current app logic and add the option to link a contract to more than one service?

Answer 1

Ok just to clarify one example:
Contract is "Cleaning of House"
Services are "Cleaning of first floor" and "Cleaning of second floor"
Invoices are "Invoice1", "Invoice2", ...
You want a relationship that "Invoice1" can be linked to "Cleaning of first floor" AND "Cleaning of second floor".
models.py
class Contract(models.Model):
"""Can hold multiple Services"""
pass
class Service(models.Model):
"""Is linked to one specific Contract"""
contract = models.ForeignKey(Contract, on_delete=models.CASCADE)
class Invoice(models.Model):
"""Can hold multiple Services and one Service can hold multiple Invoices"""
service = models.ManyToManyField(Service)
Now your question is:
"I can easily get the contract when I have the Service object, but how can I get the Service when I have the Contract object?
con = Contract.objects.all().first()
queryset = con.service_set.all() # gives you all related Services for that specific Contract
Read more about ManytoOne
And:
"How can I get the Invoice when I have the Service object?"
ser = Service.objects.all().first()
queryset = ser.invoice_set.all() # gives you all related Invoices for that specific Service
Read more about ManyToMany
Let me know how it goes

Answer 2

Thanks to #Tarquinius for your help – I found a solution based on his suggestion. The three models in question are connected as follows:
class Service(models.Model):
pass
class Contract(models.Model):
services = models.ManyToManyField(Service)
class Invoice(models.Model):
contract = models.ForeignKey(Contract, on_delete=models.CASCADE)
The query quoted above did not even need to be modified (apart from the different fieldname) to reflect the possibility of more than one service per contract, I just had to add the distinct() function:
invoices = Invoice.objects.filter(models.Q(contract__services__building=self.tenancy_agreement.unit.building), models.Q(begin__lte=self.begin, end__gt=self.begin) | models.Q(begin__gt=self.begin, begin__lt=self.end)).distinct()
In hindsight, this is quite obvious (and simple).