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","description":"product 1","instock":120},{"_id":2,"sku":"bread","description":"product 2","instock":80},{"_id":3,"sku":"cashews","description":"product 3","instock":60},{"_id":4,"sku":"pecans","description":"product 4","instock":70},{"_id":5,"sku":null,"description":"Incomplete"},{"_id":6}]}

db.orders.aggregate([{$unionWith:{coll:"inventory",pipeline:[{$addFields:{"fromInventory":1}}]}},{"$group":{"_id":"$fromInventory","content":{"$addToSet":"$$ROOT"}}},{"$project":{"content":1,_id:0}}])