db={"users":[{"username":"John"},{"username":"Doe"}],"messages":[{"title":"Lorem Ipsum!","views":400,"author":"John"},{"title":"Lorem Ipsum!","views":200,"author":"John"},{"title":"Lorem Ipsum!","views":100,"author":"John"},{"title":"Lorem Ipsum!","views":403,"author":"Doe"},{"title":"Lorem Ipsum!","views":299,"author":"Doe"}]}

db.users.aggregate([{"$lookup":{"from":"messages","localField":"username","foreignField":"author","as":"highest_views"}},{"$set":{"highest_views":{"$max":"$highest_views.views"}}}])