A ModelSerializer that allows for optional fields to be added to a serializer. These fields only appear when they are referred to by name with a `include` GET parameter. Additionally, a user can provide a `fields` GET parameter to limit which fields are serialized for output.

serializer_mixins.py

from rest_framework import serializers


class DynamicFieldsModelSerializer(serializers.ModelSerializer):
    """
    A ModelSerializer that allows for optional fields to be added to a
    serializer.  These fields only appear when they are referred to by
    name with a `include` GET parameter.

    Additionally, a user can provide a `fields` GET parameter to limit
    which fields are serialized for output.
    """

    def __init__(self, *args, **kwargs):
        fields = getattr(self.Meta, 'fields', None) or self.get_fields().keys()
        optional_fields = getattr(self.Meta, 'optional_fields', False)

        # Combine fields and optional fields into fields attribute
        if optional_fields:
            if not isinstance(self.Meta.optional_fields, (list, tuple)):
                raise Exception("'optional_fields' must be list or tuple")
            self.Meta.fields = type(optional_fields)(fields) + self.Meta.optional_fields

        # Instantiate the superclass normally
        super(DynamicFieldsModelSerializer, self).__init__(*args, **kwargs)

        # Don't apply field-limiting logic when deserializing data
        request = self.context.get('request')
        if not request:
            return
        fields = request.GET.getlist('fields')
        included = request.GET.getlist('include')

        # Drop any fields that are not specified in the `fields` argument.
        if fields:
            # Accept either comma-separated values or multiple GET params with same name
            fields = [f for f in fields for f in f.split(',')]
            allowed = set(fields)
            existing = set(self.fields.keys())
            for field_name in existing - allowed:
                self.fields.pop(field_name)

        # Omit any optional fields that aren't specified in GET param
        elif optional_fields:
            # Accept either comma-separated values or multiple GET params with same name
            included = [f for f in included for f in f.split(',')]
            for optional_field in optional_fields:
                if optional_field not in included:
                    self.fields.pop(optional_field)