db={"methods":[{"_id":1,"name":"Test","reagents":[{_id:1,quantity:2},{_id:2,quantity:4}]}],"items":[{"_id":1,"name":"ItemOne","other":"field"},{"_id":2,"name":"ItemTwo","other":"field"}]}

db.methods.aggregate([{$lookup:{from:"items",localField:"reagents._id",foreignField:"_id",as:"reagent_items"}}])