group by date in mogodb query without considering

问题:

I have the following code:

DBObject groupFields = new BasicDBObject("_id", "$Date");
    groupFields.put("count", new BasicDBObject("$sum", 1));
    DBObject groupBy = new BasicDBObject("$group", groupFields);
    stages.add(groupBy);
    DBObject project = new BasicDBObject("_id", 0);
    project.put("count", 1);
    project.put("Date", "$_id");
    stages.add(new BasicDBObject("$project", project));

    DBObject sort = new BasicDBObject("$sort", new BasicDBObject("Date", 1));
    stages.add(sort);

to group by Date and show the count which works fine but the problem is my date is in the following format:

"Date" : ISODate("2014-11-19T04:11:22.000Z")

and as you can see it has time as well but I want to group by date without considering time so I want to consider my date like this:
2014-11-19T04:00:00.000Z but I do not know how to do that ...is there any mongodb function like date in mysql to do that?

Can anyone help?

Update:

I think this might be helpful but still do not know how to do it: link that might help

回答1:

The solution depends on whether you need to have the result "Date" as ISODate() type or if it's okay to get it back as a string.

If you are okay with string, new version of MongoDB (3.0) introduced $dateToString projection operator.

If it has to be ISODate() you can use the trick I describe in details in this blog post: http://www.kamsky.org/stupid-tricks-with-mongodb/stupid-date-tricks-with-aggregation-framework