mongodb里的group语句

data = [{uid: 1 , event: 'a' , time: 1} , 
        {uid: 1 , event: 'b' , time: 2} ,
        {uid: 2 , event: 'c' , time: 2} ,
        {uid: 3 , event: 'd' , time: 4}
       ]
 { {uid: 1} : [{uid: 1 , event: 'a' , time: 1} , {uid: 1 , event: 'b' , time: 2} ],
   {uid: 2} : [{uid: 2 , event: 'c' , time: 2} ],
   {uid: 3} : [{uid: 3 , event: 'd' , time: 4} ] }
You needn't use the reduce function to actually reduce anything

>>> coll.insert(dict(uid=1,event='a',time=1))
    ObjectId('4d5b91d558839f06a8000000')
>>> coll.insert(dict(uid=1,event='b',time=2))
    ObjectId('4d5b91e558839f06a8000001')
>>> coll.insert(dict(uid=2,event='c',time=2))
    ObjectId('4d5b91f358839f06a8000002')
>>> coll.insert(dict(uid=3,event='d',time=4))
    ObjectId('4d5b91fd58839f06a8000003')
>>> result = coll.group(['uid'], None,{'list': []}, 'function(obj, prev) {prev.list.push(obj)}')
>>> len(result)     # will show three groups
    3
>>> int(result[0]['uid'])
    1
>>> result[0]['list']
    [{u'event': u'a', u'_id': ObjectId('4d5b...0000'), u'uid': 1, u'time': 1},
     {u'event': u'b', u'_id': ObjectId('4d5b...0001'), u'uid': 1, u'time': 2}]
>>> int(result[1]['uid'])
    2
>>> result[1]['list']
    [{u'event': u'c', u'_id': ObjectId('4d5b...0002'), u'uid': 2, u'time': 2}]
>>> int(result[2]['uid'])
    3
>>> result[2]['list']
    [{u'event': u'd', u'_id': ObjectId('4d5b...0003'), u'uid': 3, u'time': 4}]