Aggregation agg = Aggregation.newAggregation(
Aggregation.match(new Criteria().orOperator(new Criteria("to").is(ukey), new Criteria().and("fromAccount").is(ukey))),
Aggregation.sort(Sort.Direction.DESC,"_id"),
Aggregation.group("to","fromAccount")
);
Aggregation.group 要排在Aggregation.match后面,否则结果集不准确。