models.py

def _render_ref_no(self, template):
        """
        Renders template provided in the string.Template format. Uses output of *TemplateSerializer as the template's
        context.
        """
        ref_no_template = Template(template)

        from legal.api.serializers import MatterTemplateSerializer, SubMatterTemplateSerializer
        srlzr = MatterTemplateSerializer if isinstance(self, Matter) else SubMatterTemplateSerializer

        def flatten(data, prefix=''):
            """
            Flattens nested dicts. For Example:
            {'title': 'lala', 'matter_type': {'title': 'Patent', 'code': 'P'}} ->
            {'title': 'lala', 'matter_type_title': 'Patent', 'matter_type_code': 'P'}
            Dicts can be nested arbitrary deep.
            """
            result = {}
            for key in data.keys():
                if isinstance(data[key], dict):
                    result.update(flatten(data[key], prefix + key + '_'))
                else:
                    result[prefix + key] = data[key]
            return result

        return ref_no_template.substitute(flatten(srlzr(self).data))