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

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?

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
        }
      }
]);

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"
    }
}]);

Just to add a little thing to the previous answer: You can put a 0 to a project item that you want to ignore and the rest will be retrieved, so you don´t need to write all the list with 1:

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": {
      "campaign.client.shhh": 0
    }
  }
])