'Invoke the compute method on ``records``.'

def _compute_value(self, records):

    fields = records._field_computed[self]
    for field in fields:
        for record in records:
            record._cache[field] = field.convert_to_cache(False, record, validate=False)
    if isinstance(self.compute, basestring):
        getattr(records, self.compute)()
    else:
        self.compute(records)
