db={"orders":[{"_id":1,"item":"almonds","price":12,"quantity":2},{"_id":2,"item":"pecans","price":20,"quantity":1},{"_id":3}],"inventory":[{"_id":1,"sku":"almonds","orderId":1,"description":"product 1","instock":120},{"_id":2,"sku":"bread","orderId":1,"description":"product 2","instock":80},{"_id":20,"sku":"almondsbread","orderId":1,"description":"product 2","instock":80},{"_id":3,"sku":"cashews","orderId":1,"description":"product 3","instock":60},{"_id":4,"sku":"pecans","orderId":2,"description":"product 4","instock":70},{"_id":5,"sku":null,"orderId":2,"description":"Incomplete"},{"_id":6}]}

db.inventory.aggregate([{"$lookup":{"from":"orders","localField":"orderId","foreignField":"_id","as":"order_docs"}},{"$unwind":"$order_docs"},{"$match":{"order_docs.item":{"$regex":"pec","$options":"i"}}}])