MongoDB aggregation with $lookup only include (or project) some fields to return from query
<p>In mongo, after doing an <code>aggregation</code> with <code>$lookup</code>, I would like the request to return only some fields and not the whole document.</p>

<p>I have the following query :</p>

<pre><code>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"
    }
}]);
</code></pre>

<p>This request will return me this :</p>

<pre><code>{
"_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" : "mail@mail.com",
        }
    ]
}
</code></pre>

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