how to use populate and aggregate in same statement?

Edit: Looks like there's a new way to do it in the latest Mongoose API (see the above answer here: https://stackoverflow.com/a/23142503/293492)

Old answer below

You can use $lookup which is similar to populate.

In an unrelated example, I use $match to query for records and $lookup to populate a foreign model as a sub-property of these records:

 Invite.aggregate( { $match: {interview: req.params.interview}}, { $lookup: {from: 'users', localField: 'email', foreignField: 'email', as: 'user'} } ).exec( function (err, invites) { if (err) { next(err); } res.json(invites); } ); 

You have to do it in two, not in one statement.

In async await scenario, make sure await until populate.

const appointments = await Appointments.aggregate([...]); await Patients.populate(appointments, {path: "patient"}); return appointments;

or (if you want to limit)

await Patients.populate(appointments, {path: "patient", select: {_id: 1, fullname: 1}});

You can do it in one query like this:

Appointments.aggregate([{ $group: { _id: '$date', patients: { $push: '$patient' } } }, { $project: { date: '$_id', patients: 1, _id: 0 } }, { $lookup: { from: "patients", localField: "patient", foreignField: "_id", as: "patient_doc" } } ]) 

populate basically uses $lookup under the hood. in this case no need for a second query. for more details check MongoDB aggregation lookup

Perform a Join with $lookup

A collection orders contains the following documents:

{ "_id" : 1, "item" : "abc", "price" : 12, "quantity" : 2 } { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1 } { "_id" : 3 } 

Another collection inventory contains the following documents:

{ "_id" : 1, "sku" : "abc", description: "product 1", "instock" : 120 } { "_id" : 2, "sku" : "def", description: "product 2", "instock" : 80 } { "_id" : 3, "sku" : "ijk", description: "product 3", "instock" : 60 } { "_id" : 4, "sku" : "jkl", description: "product 4", "instock" : 70 } { "_id" : 5, "sku": null, description: "Incomplete" } { "_id" : 6 } 

The following aggregation operation on the orders collection joins the documents from orders with the documents from the inventory collection using the fields item from the orders collection and the sku field from the inventory collection:

db.orders.aggregate([ { $lookup: { from: "inventory", localField: "item", foreignField: "sku", as: "inventory_docs" } } ]) 

The operation returns the following documents:

{ "_id" : 1, "item" : "abc", "price" : 12, "quantity" : 2, "inventory_docs" : [ { "_id" : 1, "sku" : "abc", description: "product 1", "instock" : 120 } ] } { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "inventory_docs" : [ { "_id" : 4, "sku" : "jkl", "description" : "product 4", "instock" : 70 } ] } { "_id" : 3, "inventory_docs" : [ { "_id" : 5, "sku" : null, "description" : "Incomplete" }, { "_id" : 6 } ] } 

Reference $lookup

Short answer: You can't.

Long answer: In the Aggregation Framework, the returned fields are built by you, and you're able to "rename" document properties.

What this means is that Mongoose can't identify that your referenced documents will be available in the final result.

The best thing you can do in such a situation is populate the field you want after the query has returned. Yes, that would result in two DB calls, but it's what MongoDB allows us to do.

Somewhat like this:

Appointments.aggregate([ ... ], function( e, result ) { if ( e ) return; // You would probably have to do some loop here, as probably 'result' is array Patients.findOneById( result.patient, function( e, patient ) { if ( e ) return; result.patient = patient; }); }); 

I see that there are many answers, I am new to mongoldb and I would like to share my answer too. I am using aggregate function along with lookup to populate the patients. To make it easy to read I have changed the names of the collections and fields.

Hope it's helpful.

DB:

db={ "appointmentCol": [ { _id: ObjectId("518ee0bc9be1909012000001"), date: ISODate("2013-05-13T22:00:00Z"), patientId: ObjectId("518ee0bc9be1909012000001") }, { _id: ObjectId("518ee0bc9be1909012000002"), date: ISODate("2013-05-13T22:00:00Z"), patientId: ObjectId("518ee0bc9be1909012000002") }, { _id: ObjectId("518ee0bc9be1909012000003"), date: ISODate("2013-05-13T22:00:00Z"), patientId: ObjectId("518ee0bc9be1909012000003") } ], "patientCol": [ { "_id": ObjectId("518ee0bc9be1909012000001"), "name": "P1" }, { "_id": ObjectId("518ee0bc9be1909012000002"), "name": "P2" }, { "_id": ObjectId("518ee0bc9be1909012000003"), "name": "P3" }, ] } 

Aggregate Query using lookup:

db.appointmentCol.aggregate([ { "$lookup": { "from": "patientCol", "localField": "patientId", "foreignField": "_id", "as": "patient" } } ]) 

Output:

[ { "_id": ObjectId("518ee0bc9be1909012000001"), "date": ISODate("2013-05-13T22:00:00Z"), "patient": [ { "_id": ObjectId("518ee0bc9be1909012000001"), "name": "P1" } ], "patientId": ObjectId("518ee0bc9be1909012000001") }, { "_id": ObjectId("518ee0bc9be1909012000002"), "date": ISODate("2013-05-13T22:00:00Z"), "patient": [ { "_id": ObjectId("518ee0bc9be1909012000002"), "name": "P2" } ], "patientId": ObjectId("518ee0bc9be1909012000002") }, { "_id": ObjectId("518ee0bc9be1909012000003"), "date": ISODate("2013-05-13T22:00:00Z"), "patient": [ { "_id": ObjectId("518ee0bc9be1909012000003"), "name": "P3" } ], "patientId": ObjectId("518ee0bc9be1909012000003") } ] 

Playground: mongoplayground.net

domain.Farm.aggregate({ $match: { "_id": mongoose.Types.ObjectId(farmId) } }, { $unwind: "$SelfAssessment" }, { $match: { "SelfAssessment.questionCategoryID": QuesCategoryId, "SelfAssessment.questionID": quesId } },function(err, docs) { var options = { path: 'SelfAssessment.actions', model: 'FarmAction' }; domain.Farm.populate(docs, options, function (err, projects) { callback(err,projects); }); }); 

results i got action model populate

{ "error": false, "object": [ { "_id": "57750cf6197f0b5137d259a0", "createdAt": "2016-06-30T12:13:42.299Z", "updatedAt": "2016-06-30T12:13:42.299Z", "farmName": "abb", "userId": "57750ce2197f0b5137d2599e", "SelfAssessment": { "questionName": "Aquatic biodiversity", "questionID": "3kGTBsESPeYQoA8ae2Ocoy", "questionCategoryID": "5aBe7kuYWIEoyqWCWcAEe0", "question": "Waterways protected from nutrient runoff and stock access through fencing, buffer strips and off stream watering points", "questionImage": "http://images.contentful.com/vkfoa0gk73be/4pGLv16BziYYSe2ageCK04/6a04041ab3344ec18fb2ecaba3bb26d5/thumb1_home.png", "_id": "57750cf6197f0b5137d259a1", "actions": [ { "_id": "577512c6af3a87543932e675", "createdAt": "2016-06-30T12:38:30.314Z", "updatedAt": "2016-06-30T12:38:30.314Z", "__v": 0, "Evidence": [], "setReminder": "", "description": "sdsdsd", "priority": "High", "created": "2016-06-30T12:38:30.312Z", "actionTitle": "sdsd" } ], "answer": "Relevant" }, "locations": [] } ], "message": "", "extendedMessage": "", "timeStamp": 1467351827979 } 

I used lookup instead, and it worked well. See the code snipped below.

Post.aggregate([ { $group: { // Each `_id` must be unique, so if there are multiple // posts with the same category, MongoDB will increment `count`. _id: '$category', count: { $sum: 1 } } }, //from: is collection name in MongoDB, localField are primary and foreign keys in Model. {$lookup: {from: 'categories', localField: '_id', foreignField:'_id', as: 'category'}} ]).then(categoryCount => { console.log(categoryCount); let json = []; categoryCount.forEach(cat => { console.log(json); });