Update models.py

now you can filter the related objects using the filter keyword.
Example:
        serialize(obj, fields=[
            'name',   # obj.name
            'dob',    # obj.dob
            ('jobs', dict(
                filter=Q(title='programmer'),
                fields=[
                    'title',  # job.title
                    'from',   # job.from
                    'to',     # job.to,
                    ('duration', lambda job: job.to - job.from),
                ]
            ))
        ])