db={"sites":[{"_id":3,"host":"www.example-foo.com","path":"/bar","hasVisited":false}],"webpages":[{"_id":5,"url":"https://www.example-foo.com/bar"},{"_id":8,"url":"https://www.fizz.com/buzz"}]}

db.sites.aggregate([{"$match":{"hasVisited":{"$eq":false}}},{"$limit":1},{"$set":{"url":{"$concat":["https://","$host","$path"]}}},{"$lookup":{"from":"webpages","localField":"url","foreignField":"url","as":"pages"}},{"$project":{"_id":0,"pages":1}}])