Django REST Framework custom fields validation

Question

I am trying to create custom validation for a model, to check that its start_date is before its end_date and it is proving near impossible.

Stuff I've tried:

• built-in Django validators: none check for this

• writing my own, like so:

  def validate_date(self):
     if self.start_date < self.end_date:
          raise serializers.ValidationError("End date must be after start date.")
  

That bit of code I have added to the Serializer class (and then the model), but it does not seem to get called in either location.

I also found this bit of code that might be of use, but I don't know how to integrate in my method- it seems that it would work to validate one model attribute, but I need to check between two attributes.

My model:

class MyModel(models.Model):

    created = models.DateTimeField(auto_now_add=True)
    relation_model = models.ForeignKey(RelationModel, related_name="mymodels")
    priority = models.IntegerField(
        validators = [validators.MinValueValidator(0), validators.MaxValueValidator(100)])
    start_date = models.DateField()
end_date = models.DateField()

    @property
    def is_active(self):
        today = datetime.date.today()
        return (today >= self.start_date) and (today <= self.end_date)

    def __unicode__(self):
        ...

    class Meta:
        unique_together = ('relation_model', 'priority', 'start_date', 'end_date')

Fyi, all the other validations work!

My serializer:

class MyModelSerializer(serializers.ModelSerializer):

    relation_model = RelationModelSerializer
    is_active = serializers.Field(source='is_active')

    def validate_date(self):
        if self.start_date > self.end_date:
            raise serializers.ValidationError("End date must be after start date.")   

    class Meta:
        model = MyModel
        fields = (
            'id', 'relation_model', 'priority', 'start_date', 'end_date', 'is_active'
        )

My view:

class MyModelList(generics.ListCreateAPIView):
    permission_classes = (IsAdminUser,)
    queryset = MyModel.objects.all()
    serializer_class = MyModelSerializer
    ordering = ('priority')

Answer 1

You should use an object wide validation (validate()), since validate_date will never be called since date is not a field on the serializer. From the documentation:

class MySerializer(serializers.ModelSerializer):
    def validate(self, data):
        """
        Check that the start is before the stop.
        """
        if data['start_date'] > data['end_date']:
            raise serializers.ValidationError("finish must occur after start")
        return data

As suggested by Michel Sabchuk you can add the validation error to the end_date field:

class MySerializer(serializers.ModelSerializer):
    def validate(self, data):
        """
        Check that the start is before the stop.
        """
        if data['start_date'] > data['end_date']:
            raise serializers.ValidationError({"end_date": "finish must occur after start"})
        return data

Another possibility is to create a validator. I created one based on the code for UniqueTogetherValidator:

from rest_framework.utils.representation import smart_repr

class DateBeforeValidator:
    """
    Validator for checking if a start date is before an end date field.
    Implementation based on `UniqueTogetherValidator` of Django Rest Framework.
    """
    message = _('{start_date_field} should be before {end_date_field}.')

    def __init__(self, start_date_field="start_date", end_date_field="end_date", message=None):
        self.start_date_field = start_date_field
        self.end_date_field = end_date_field
        self.message = message or self.message

    def __call__(self, attrs, serializer):
        if attrs['start_date'] > attrs['end_date']:
            message = self.message.format(
                start_date_field=self.start_date_field,
                end_date_field=self.end_date_field,
            )
            # Replace the following line with
            #   raise serializers.ValidationError(
            #       {self.end_date_field: message},
            #       code='date_before',
            #   )
            # if you want to raise the error on the field level
            raise serializers.ValidationError(message, code='date_before')

    def __repr__(self):
        return '<%s(start_date_field=%s, end_date_field=%s)>' % (
            self.__class__.__name__,
            smart_repr(self.start_date_field),
            smart_repr(self.end_date_field)
        )


class MySerializer(serializers.ModelSerializer):
    class Meta:
        # If your start/end date fields have another name give them as kwargs tot the
        # validator:
        #   DateBeforeValidator(
        #       start_date_field="my_start_date", 
        #       end_date_field="my_end_date",
        #   )
        validators = [DateBeforeValidator()]

Pre DRF 3.0 you could also add it to the clean function of a model, but this is not called anymore in DRF 3.0.

class MyModel(models.Model):
    start_date = models.DateField()
    end_date = models.DateField()
    def clean(self):
        if self.end_date < self.start_date:
            raise ValidationError("End date must be after start date.")