"Incorrect type. Expected pk value, received str."
    ]

class Case(models.Model):
    name = models.CharField(max_length=200)
    task = models.ManyToManyField('task.Task',blank=True, null=True)
    assigned_to = models.ForeignKey("account.User",null=True, blank=True, on_delete=models.SET_NULL)

class CaseSerializer(serializers.ModelSerializer):
    class Meta:
        model = Case
        fields = ('id', 'name', 'task', 'assigned_to')