@staticmethod
def _nested_lookup(doc, field):
    """
        Performs a nested lookup of doc using a period (.) delimited
        list of fields.  This is a nested dictionary lookup.
        :param doc: document to perform lookup on
        :param field: period delimited list of fields
        :return:
        """
    value = doc
    keys = field.split('.')
    try:
        for k in keys:
            value = value[k]
    except KeyError:
        return None
    return str(value)
