Custom validation of ModelForm in Django - django

Question

I am working on a Django app to register sales. I have created three models: Project, Employee, and Sale.
The project and employee models are as follows:
class Project(models.Model):
project_id = models.IntegerField(primary_key = True)
name = models.CharField(max_length = 100, unique = True)
class Meta:
ordering = ['name']
def __str__(self):
return self.name
class Employee(models.Model):
employee_id = models.IntegerField(primary_key = True)
name = models.CharField(max_length = 50)
email = models.CharField(max_length = 40)
class Meta:
ordering = ['name']
def __str__(self):
return self.name
Then the sale model:
class Sale(models.Model):
sale_name = models.CharField(max_length = 30)
project = models.ForeignKey('Project', on_delete = models.CASCADE)
proactive_seller = models.ManyToManyField(Employee, related_name = 'proactive')
participants = models.ManyToManyField(Employee, related_name = 'participant')
doers = models.ManyToManyField(Employee, related_name = 'doer')
start_date = models.DateField()
end_date = models.DateField()
def __str__(self):
return self.sale_name
So the each sale object contains information on what project the sale is related to, which employee was the proactive/lead seller, which employees were participating in the sale, and also which employees will be doing the actual project.
In my forms.py I want to make sure that the sales are unique in the sense that I want to raise an error if the user is trying to enter a sale which already has the same project, same date and the same doers, i.e. the doers can't be allocated to the project more than once at a time.
My forms.py is currently looking like this:
class SaleForm(ModelForm):
class Meta:
model = Sale
widgets = {
'start_date': DatePickerInput(),
'end_date': DatePickerInput(),
}
I tried the following:
def clean(self):
cleaned_data = super.clean()
start = cleaned_data.get('start_date')
end = cleaned_data.get('end_date')
doers = cleaned_data.get('doers')
project = cleaned_data.get('project')
if start and end and doers and project:
queryset = Sale.objects.all()
# Filter based on project
q = queryset.filter(project__name=project, start_date = start, end_date = end)
for employee in doers:
q = q.filter(doers__name=employee)
if q.count() > 1:
raise forms.ValidationError('Sale has already been registered.')
However, the validation does not work as expected: I am still allowed to allocate “employees” to the same “project” at the same time (ie start date and end date).
Help is much appreciated.

Answer 1

What you're attempting to do is validate every instance of the M2M relationship for a given instance. This can be fairly difficult to do. What should suffice is to filter down on sales with the same number of doers and filter out sales that contain a different doer on them.
from django.db.models import F, OuterRef, Exists, Q
q = queryset.filter(project__name=project, start_date=start, end_date = end)
other_doers = Employee.objects.filter(
# Exclude any employee with the name of the doers on this project.
# We only want other doers.
~Q(name__in=[e.name for e in doers]),
# This links the subquery to the main query (Sale)
doer=OuterRef('id'),
)
q = q.annotate(
# Get the count of doers per sale
doer_count=Count('doers__id', distinct=True),
# Check if other doers are in the project
has_other_doer=Exists(other_doers)
).filter(
# Only look for sales with the same number of doers
doer_count=len(doers),
# Filter out sales that contain other doers
has_other_doer=False,
)