自问自答: SELECT temp.id, temp.time, temp.ttable FROM ((SELECT id, time, '1' ttable FROM table_1 ORDER BY time DESC LIMIT 5) UNION ALL (SELECT id, time, '2' ttable FROM table_2 ORDER BY time DESC LIMIT 5)) AS temp ORDER BY time DESC LIMIT 5

我真煞笔，直接写代码能执行，想放哪放哪。

看了官方模块的readme文档， 需要先 var ObjectID = require('mongodb').ObjectID; 然后再collection.findAndRemove({_id: new ObjectID(id)}）这样才可以

自己写了一个， str = str.replace(/^\[(.*)\]$/, "\{$1\}")