MongoDB aggregation with $lookup only include (or project) some fields to return from query

Question

In mongo, after doing an aggregation with $lookup, I would like the request to return only some fields and not the whole document.

I have the following query :

db.somecollection.aggregate([{     $lookup: {         from: "campaigns",         localField: "campId",         foreignField: "_id",         as: "campaign"     } }, {     $unwind: "$campaign" }, {     $lookup: {         from: "entities",         localField: "campaign.clientid",         foreignField: "_id",         as: "campaign.client"     } }]); 

This request will return me this :

{ "_id" : ObjectId("56cc7cd1cc2cf62803ebfdc7"), "campId" : ObjectId("56c740e4479f46e402efda84"), "articleId" : ObjectId("56c742c06094640103ba3843"), "campaign" : {     "_id" : ObjectId("56c740e4479f46e402efda84"),     "clientid" : ObjectId("56c740b8479f46e402efda83"),     "client" : [         {             "_id" : ObjectId("56c740b8479f46e402efda83"),             "username" : "someusername",             "shhh" : "somehashedpassword",             "email" : "[email protected]",         }     ] } 

The request works well, but I would like to filter the fields in campaign.client to only get for example _id and username. Is there a way to do this in a MongoDB aggregate request?

Answer 1

Just to help others with this, @SiddhartAjmera has the right answer, I only needed to add double quotes for nested values like "campaign.clientid".

The final code should be:

db.somecollection.aggregate([       {         "$lookup": {           "from": "campaigns",           "localField": "campId",           "foreignField": "_id",           "as": "campaign"         }       },       {         "$unwind": "$campaign"       },       {         "$lookup": {           "from": "entities",           "localField": "campaign.clientid",           "foreignField": "_id",           "as": "campaign.client"         }       },       {         "$project": {           "_id": 1,           "campId": 1,           "articleId": 1,           "campaign._id": 1,           "campaign.clientid": 1,           "campaign.client._id": 1,           "campaign.client.username": 1         }       } ]); 

Answer 2

Using pipeline and $project inside $lookup

db.somecollection.aggregate([{     $lookup: {         from: "campaigns",         localField: "campId",         foreignField: "_id",         as: "campaign"     } }, {     $unwind: "$campaign" }, {     $lookup: {         from: "entities",         let: { client_id: "$campaign.clientid" },             pipeline : [             { $match: { $expr: { $eq: [ "$_id", "$$client_id" ] } }, },             { $project : { _id:1, username:1 } }         ],         as: "campaign.client"     } }]);