How do I use aggregation operators in a $match in MongoDB (for example $year or $dayOfMonth)?

Question

I have a collection full of documents with a created_date attribute. I\'d like to send these documents through an aggregation pipeline to do some work on them. Ideally I would

Answer 1

Try this;

db.createCollection("so");
db.so.remove();
db.so.insert([
{
    post_body: 'This is the body of test post 1',
    created_date: ISODate('2012-09-29T05:23:41Z'),
    comments: 48
},
{
    post_body: 'This is the body of test post 2',
    created_date: ISODate('2012-09-24T12:34:13Z'),
    comments: 10
},
{
    post_body: 'This is the body of test post 3',
    created_date: ISODate('2012-08-16T12:34:13Z'),
    comments: 10
}
]);
//db.so.find();

db.so.ensureIndex({"created_date":1});
db.runCommand({
    aggregate:"so",
    pipeline:[
        {
            $match: { // filter only those posts in september
                created_date: { $gte: ISODate('2012-09-01'), $lt: ISODate('2012-10-01') }
            }
        },
        {
            $group: {
                _id: null, // no shared key
                comments: { $sum: "$comments" } // total comments for all the posts in the pipeline
            }
        },
]
//,explain:true
});

Result is;

{ "result" : [ { "_id" : null, "comments" : 58 } ], "ok" : 1 }

So you could also modify your previous example to do this, although I'm not sure why you'd want to, unless you plan on doing something else with month and year in the pipeline;

{
    aggregate: 'posts',
    pipeline: [
     {$match: { created_date: { $gte: ISODate('2012-09-01'), $lt: ISODate('2012-10-01') } } },
     {$project:
          {
               month : {$month:'$created_date'},
               year : {$year:'$created_date'}
          }
     },
     {$match:
          {
               month:9,
               year: 2012
           }
     },
     {$group:
         {_id: '0',
          totalComments:{$sum:'$comments'}
         }
      }
    ]
 }