db={"collA":[{"Sku":"A","Stock":[{"StoreCode":"1","Qty":3}]},{"Sku":"B","Stock":[{"StoreCode":"1","Qty":3},{"StoreCode":"2","Qty":5},{"StoreCode":"10","Qty":7}]}],"collB":[{"Sku":"A","MinQty":2},{"Sku":"B","MinQty":5}]}

db.collA.aggregate([{"$lookup":{"from":"collB","localField":"Sku","foreignField":"Sku","as":"skus"}},{"$project":{"_id":0,"Sku":1,"Availability":{"$cond":{"if":{"$gte":[{"$min":"$Stock.Qty"},{"$max":"$skus.MinQty"}]},"then":"in stock","else":"out of stock"}}}}])