db={"support":[{_id:1,title:"Support",instructions:[{image:"image-id-string-one"},{image:"image-id-string-two"},{image:"image-id-string-three"}]}],"images":[{_id:11,id:"image-id-string-one",image:"actual-image-one"},{_id:12,id:"image-id-string-two",image:"actual-image-two"},{_id:13,id:"image-id-string-three",image:"actual-image-three"}],"meta":[{"by":"steevej","at":"gmail"}]}

db.support.aggregate([{"$match":{"title":"Support"}},{"$lookup":{"from":"images","localField":"instructions.image","foreignField":"id","as":"instructions"}},{"$set":{"instructions":{"$map":{"input":"$instructions","in":{"image":"$$this.image"}}}}}])