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

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
        }
      }
]);