unique_together does not prevent duplicate records - django

Question

I have two models like this :
class Preference(models.Model):
lat = models.DecimalField(max_digits=25,decimal_places=15)
lng = models.DecimalField(max_digits=25,decimal_places=15)
name = models.CharField(max_length=150,null=True)
address = models.TextField(max_length=350,null=True)
type = models.CharField(max_length=150,blank=True,null=True)
class PrefenceOfUser(models.Model):
user = models.ForeignKey(get_user_model(), on_delete=models.CASCADE)
place_detail = models.ForeignKey(Preference, on_delete=models.CASCADE)
def __str__(self):
return self.user.username
class Meta:
unique_together = ('user', 'place_detail',)
this is the json i post to my apiview :
{
"lat": "29.621142463088336",
"lng": "52.520185499694527",
"name":"cafesama1",
"address":"streetn1",
"type":"cafe"
}
in views.py :
class PreferedLocationsOfUsers(APIView):
def post(self, request, format=None):
serializer = PreferLocationSerializer(data=request.data)
if serializer.is_valid():
location= Preference(**serializer.data)
location.save()
user_perefeces = PrefenceOfUser(user=request.user,place_detail=location)
user_perefeces.save()
return Response({'location saved'},status=status.HTTP_201_CREATED)
return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
i want to prevent the user to save dublicated records in database but when location object is saved in PrefenceOfUser unique_together does not prevent dublicating. any idea why?

Answer 1

You do have a migration issue as per the comments. Get rid of the duplicates, re-run the migration until you get no errors and you're fine.
However, I would turn this around as a model design problem. You shouldn't need to resort to manual constraints for this.
class Preference(models.Model):
lat = models.DecimalField(max_digits=25,decimal_places=15)
lng = models.DecimalField(max_digits=25,decimal_places=15)
name = models.CharField(max_length=150,null=True)
address = models.TextField(max_length=350,null=True)
type = models.CharField(max_length=150,blank=True,null=True)
users = models.ManyToManyField(get_user_model(), blank=True)
This above code will have exactly the same implications as yours, is cleaner and more Djangoesque.
You can still access what you call PrefenceOfUser through Preference.users.through. If you plan to add more attributes to the selection (ie. when did user add their preference), you can just do:
class PrefenceOfUser(models.Model):
user = models.ForeignKey(get_user_model(), on_delete=models.CASCADE)
place_detail = models.ForeignKey(Preference, on_delete=models.CASCADE)
extra_field = models.WhateverField()
def __str__(self):
return self.user.username
and change the Preference.users to
models.ManyToManyField(get_user_model(), through=PrefenceOfUser)
which will still still ensure the uniqueness.