Django REST Framework custom fields validation

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):
        if attrs[self.start_date_field] > attrs[self.end_date_field]:
            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.")