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,"disabled":true},{"_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([{$lookup:{from:"inventory",let:{item:"$item"},pipeline:[{$match:{$and:[{$expr:{$eq:["$sku","$$item"]}},{disabled:{$exists:false}}]}}],as:"inv"}}])