'Get the last update meta-data of the current record
@return: a dict {modified_by: <user>, modified_on: <datestr>},
depending on which of these attributes are available
in the current record'

def last_update(self):

    output = {}
    record_id = self.record_id
    if record_id:
        record = None
        fields = []
        table = self.table
        if ('modified_on' in table.fields):
            fields.append(table.modified_on)
        if ('modified_by' in table.fields):
            fields.append(table.modified_by)
        if fields:
            query = (table._id == record_id)
            record = current.db(query).select(limitby=(0, 1), *fields).first()
        if record:
            T = current.T
            if ('modified_by' in record):
                if (not record.modified_by):
                    modified_by = T('anonymous    user')
                else:
                    modified_by = table.modified_by.represent(record.modified_by)
                output['modified_by'] = (T('by    %(person)s') % {'person': modified_by})
            if ('modified_on' in record):
                modified_on = S3DateTime.datetime_represent(record.modified_on, utc=True)
                output['modified_on'] = (T('on    %(date)s') % {'date': modified_on})
    return output
