按ObjectId日期聚合MongoDB结果

Question

如何通过ObjectId日期汇总我的MongoDB结果.例:

默认光标结果:

cursor = [
    {'_id': ObjectId('5220b974a61ad0000746c0d0'),'content': 'Foo'},
    {'_id': ObjectId('521f541d4ce02a000752763a'),'content': 'Bar'},
    {'_id': ObjectId('521ef350d24a9b00077090a5'),'content': 'Baz'},
]

预计结果:

projected_cursor = [
    {'2013-09-08':
        {'_id': ObjectId('5220b974a61ad0000746c0d0'),'content': 'Foo'},
        {'_id': ObjectId('521f541d4ce02a000752763a'),'content': 'Bar'}
    },
    {'2013-09-07':
        {'_id': ObjectId('521ef350d24a9b00077090a5'),'content': 'Baz'}
    }
]

这就是我目前在PyMongo中用来实现这些结果的方法,但它很混乱,我想看看如何使用MongoDB的聚合框架(甚至MapReduce)来实现它:

cursor = db.find({}, limit=10).sort("_id", pymongo.DESCENDING)
messages = [x for x in cursor]
this_date = lambda x: x['_id'].generation_time.date()
dates = set([this_date(message) for message in messages])
dates_dict = {date: [m for m in messages if this_date(m) == date] for date in dates}

是的,我知道最简单的方法是简单地为每条记录添加一个新的日期字段,然后再按此汇总,但这不是我现在想要做的.

谢谢!

Answer 1

没有办法用mongodb的聚合框架来完成你所要求的,因为没有聚合运算符可以将ObjectId变成类似日期的东西(尽管有JIRA票证).您应该能够使用map-reduce完成您想要的任务,但是:

// map function
function domap() {
    // turn ObjectId --> ISODate
    var date = this._id.getTimestamp();
    // format the date however you want
    var year = date.getFullYear();
    var month = date.getMonth();
    var day = date.getDate();

    // yields date string as key, entire document as value
    emit(year+"-"+month+"-"+day, this);
}

// reduce function
function doreduce(datestring, docs) {
    return {"date":datestring, "docs":docs};
}

Answer 2

llovett指出的Jira Ticket已得到解决，因此现在您可以使用和等日期运算符从$isoWeek$yearObjectId.

您的聚合看起来像这样：

{
    "$project":
        {

            "_id": {
                "$dateFromParts" : {
                    "year": { "$year": "$_id"},
                    "month": { "$month": "$_id"},
                    "day": { "$dayOfMonth": "$_id"}
                }
            }
        }
}