db={"orders":[{"_id":1,"items":[{_id:11},{_id:12}]}],"inventory":[{"_id":11,"sku":"almonds","description":"product 1","instock":120},{"_id":12,"sku":"bread","description":"product 2","instock":80},{"_id":13,"sku":"cashews","description":"product 3","instock":60},{"_id":14,"sku":"pecans","description":"product 4","instock":70},{"_id":15,"sku":null,"description":"Incomplete"}],"meta":[{"by":"steevej","at":"gmail"}]}

db.orders.aggregate([{"$lookup":{"from":"inventory","localField":"items._id","foreignField":"_id","as":"_tmp.lookup"}}])