django - "Incorrect type. Expected pk value, received str" error

Question

I my django-rest-framework I have the following models:

Basically every ride has one final destination and can have multiple middle destinations.

models.py:

class Destination(models.Model):
    name=models.CharField(max_length=30)

class Ride(models.Model):
    driver = models.ForeignKey('auth.User', related_name='rides_as_driver')
    destination=models.ForeignKey(Destination, related_name='rides_as_final_destination')
    leaving_time=models.TimeField()
    num_of_spots=models.IntegerField()
    passengers=models.ManyToManyField('auth.User', related_name="rides_as_passenger")
    mid_destinations=models.ManyToManyField(Destination, related_name='rides_as_middle_destination')

serializers.py - RideSerializer

class RideSerializer(serializers.ModelSerializer):
    driver = serializers.ReadOnlyField(source='driver.user.username')

    class Meta:
        model = Ride
        fields = ('driver', 'destination', 'leaving_time',
                  'num_of_spots', 'passengers', 'mid_destinations')
        read_only_fields = ('driver', 'passengers', 'mid_destinations')

Problem is - When I am trying to POST to /rides/ in order to add a ride - for example {destination=LA, leaving_time=19:45, num_of_spots=4}

I get error "destination":["Incorrect type. Expected pk value, received str."]}

couple of questions:

• What is this error? if I have a destination as a foreign key in the Ride model, does it mean that the destination that I am adding has to be already in the Destinations table?

• How to fix this error?

Answer 1

The issue is that you are passing the name of the related Destination object into the serializer, instead of passing the pk/id of the Destination object. So Django REST framework is seeing this and complaining, because it can't resolve LA into an object.

It sounds like you may actually be looking for a SlugRelatedField, which allows you to identify objects by a slug (LA in this case) instead of their primary keys.