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class RecipeFilter(django_filters.FilterSet):
    tags = django_filters.CharFilter(field_name='tags__slug')
    is_favorited = django_filters.BooleanFilter(
        field_name='favorite_recipe',
        method='filter_is_favorited',
    )


    def filter_is_favorited(self, queryset, name, value):
        lookup = '__'.join([name, 'isnull'])
        return queryset.filter(**{lookup: True})

    class Meta:
        model = Recipe
        fields = (
            'author', 'tags',
            'is_favorited',
        )