db={"pages":[{"_id":2}],"content":[{"_id":1,"page_id":2},{"_id":2,"page_id":1}]}

db.content.aggregate([{"$lookup":{"from":"pages","localField":"page_id","foreignField":"_id","as":"pages"}},{"$addFields":{"pages_length":{"$size":"$pages"}}},{"$match":{"pages_length":0}},{"$unset":["pages","pages_length"]}])