Project with Match in aggregate not working in mongodb

[问题]

2down votefavorite I am trying to fetch data based on some match condition. First I've tried this: Here ending_date is full date format Offer.aggregate([ { $match: { carer_id : req.params.carer_id, status : 3 } }, { $group : { _id : { year: { $year : "$ending_date" }, month: { $month : "$ending_date" }}, count : { $sum : 1 } } }], function (err, res) { if (err) ; // TODO handle error console.log(res); }); which gives me following output: [ { _id: { year: 2015, month: 11 }, count: 2 } ] Now I want to check year also, so I am trying this: Offer.aggregate([ { $project: { myyear: {$year: "$ending_date"} } }, { $match: { carer_id : req.params.carer_id, status : 3, $myyear : "2015" } }, { $group : { _id : { year: { $year : "$ending_date" }, month: { $month : "$ending_date" }}, count : { $sum : 1 } } }], function (err, res) { if (err) ; // TODO handle error console.log(res); }); which gives me following output: [] as you can see, _id has 2015 as a year, so when I match year it should be come in array. But I am getting null array. Why this? Is there any other way to match only year form whole datetime? Here is the sample data { "_id": { "$oid": "56348e7938b1ab3c382d3363" }, "carer_id": "55e6f647f081105c299bb45d", "user_id": "55f000a2878075c416ff9879", "starting_date": { "$date": "2015-10-15T05:41:00.000Z" }, "ending_date": { "$date": "2015-11-19T10:03:00.000Z" }, "amount": "850", "total_days": "25", "status": 3, "is_confirm": false, "__v": 0 } { "_id": { "$oid": "563b5747d6e0a50300a1059a" }, "carer_id": "55e6f647f081105c299bb45d", "user_id": "55f000a2878075c416ff9879", "starting_date": { "$date": "2015-11-06T04:40:00.000Z" }, "ending_date": { "$date": "2015-11-16T04:40:00.000Z" }, "amount": "25", "total_days": "10", "status": 3, "is_confirm": false, "__v": 0 }

[回答]

You forgot to project the fields that you're using in $match and $group later on. For a quick fix, use this query instead: Offer.aggregate([ {     $project: {         myyear: { $year: "$ending_date" },         carer_id: 1,         status: 1,         ending_date: 1     } }, {     $match: {         carer_id: req.params.carer_id,         myyear: 2015,         status: 3     } }, {     $group: {         _id: {             year: { $year: "$ending_date" },             month: { $month: "$ending_date" }         },         count: { $sum: 1 }     } }], function (err, res) {     if (err) {} // TODO handle error     console.log(res); }); That said, Blakes Seven explained how to make a better query in her answer. I think you should try and use her approach instead.

也就是说,project的字段中必须包含match中用到的字段.

来自: https://*.com/questions/33752817/project-with-match-in-aggregate-not-working-in-mongodb