Comment on: "manual/reference/operator/aggregation/project"

I really need to combine somehow the $project aggregation with the $lookup collection.
Basically I'm returning every projects from the database, and I'm using the $project aggregator there to limit the field numbers. And i'm using a $lookup to assign the media files to these project, then filter the media fields, but I just can't find the information how can i do this. Nothing on google, stackoverflow - also no response - simply just nothing. Am I missing the logic or something? Actually joining the medias with the projects are pretty easy, but filtering out the not neccessary fields is seems impossible to me in mongoDB.