db={"users":[{username:"Testuser",events:{favorited:["abcdefg12345678","hijklmno12345678"]}}],"events":[{_id:"abcdefg12345678",name:"Party One",address:"Party St"},{_id:"hijklmno12345678",name:"Party Two",address:"Party Lane"},{_id:"pqrstuvw12345678",name:"Party Three",address:"Party Town"}]}

db.users.aggregate([{$lookup:{from:"events",localField:"events.favorited",foreignField:"_id",as:"events.favorited"}},{$project:{"events.favorited.address":0}}])