How to prevent current user from rating himself using Django model constraints - django

Question

So I have a user model and each user should be able to rate other users but shouldn't be able to rate themselves.
Model for rating is
#Rating field in User model
ratings = models.ManyToManyField('self', through='Rating',
symmetrical=False,
related_name='rated_by')
#Rating model
class Rating(models.Model):
rating = models.IntegerField(validators=[validate_rating])
from_user = models.ForeignKey(User, related_name="from_people", on_delete=models.CASCADE)
to_user = models.ForeignKey(User, related_name="to_people", on_delete=models.CASCADE)
Is there a way I can make this functionality? I was thinking django model constraints will work but I do not know how to go about it. How do I filter a request to grab the specific user to prevent them from rating themselves?
I tried:
class Rating(models.Model):
rating = models.IntegerField(validators=[validate_rating])
from_user = models.ForeignKey(User, related_name="from_people",
on_delete=models.CASCADE)
to_user = models.ForeignKey(User, related_name="to_people",
on_delete=models.CASCADE)
class Meta:
constraints = [
CheckConstraint(
check = Q(from_user != User,
to_user != User),
name = 'check_user',
)
]
but I get a Metacheck = Q(from_user != User, NameError: name 'from_user' is not defined as an error

Answer 1

You can add a database level check constraint to your model that will prevent the Rating model from having a record with the from_user and to_user the same.
from django.db.models import Q, F, CheckConstraint
class Rating(models.Model):
rating = models.IntegerField(validators=[validate_rating])
from_user = models.ForeignKey(User, related_name="from_people", on_delete=models.CASCADE)
to_user = models.ForeignKey(User, related_name="to_people", on_delete=models.CASCADE)
class Meta:
constraints = [
CheckConstraint(check=~Q(from_user=F('to_user')), name='no_self_rating')
]
Django docs constraint reference.
The above adds the constraint at the database level, but does not do any form validation.
You also ask
How do I filter a request to grab the specific user to prevent them from rating themselves?
It's not clear what you mean here. The request object passed to the view function has a user attribute that is the user using the application. I'm not sure what your view looks like but:
def my_view(request):
request.user # This is the user making the action
... # Other view code
return HttpResponse(...)
You can pass request.user to a form and validate the user is not rating themselves.

Answer 2

A negative CheckConstraint should do what you want.
from django.db import models
class Rating(models.Model):
rating = models.IntegerField(validators=[validate_rating])
from_user = models.ForeignKey(User, related_name="from_people", on_delete=models.CASCADE)
to_user = models.ForeignKey(User, related_name="to_people", on_delete=models.CASCADE)
class Meta:
constraints = [
models.CheckConstraint(
check=~models.Q(from_user=models.F('to_user')),
name='users_cannot_rate_themselves'
),
]