关于mongodb的aggregate()中$slice的使用

Question

在mongodb中使用aggregate(函数作聚合)，pipe是这样写的：

pipe = [
    {'$match':{'time':'03.02'}},
    {'$group':{'_id':{'$slice':['$address',0,1]},'counts':{'$sum':1}}},
    {'$sort':{'counts':-1}}
]

address字段是list型，然后想取出address中第一项，出现一个就加一，但是总是报错：

OperationFailure: command SON([('aggregate', u'item_info_3'), ('pipeline', [{'$match': {'time': '03.02'}}, {'$group': {'counts': {'$sum': 1}, '_id': {'$slice': ['$address', 0, 1]}}}, {'$sort': {'counts': -1}}]), ('cursor', {})]) on namespace GanjiData.$cmd failed: exception: invalid operator '$slice'

不知道是为什么，请高手看看，不胜感激！

Answer 1

I didn’t know you mongodb的版本是多少，据mongodb docs上显示，aggregation的$slice是在mongodb 3.2 joined later.

See
https://docs.mongodb.org/manual/reference/operator/aggregation/slice/