lostjohnny

    db.orders.aggregate([
       {
          $lookup:
             {
               from: "warehouses",
               let: { order_item: "$item", order_qty: "$ordered" },
               pipeline: [
                  { $match:
                     { $expr:
                        { $and:
                           [
                             { $eq: [ "$stock_item",  "$$order_item" ] },
                             { $gte: [ "$instock", "$$order_qty" ] }
                           ]
                        }
                     }
                  },
                  { $project: { stock_item: 0, _id: 0 } }
               ],
               as: "stockdata"
             }
        }
    ])

    SELECT *, stockdata
    FROM orders
    WHERE stockdata IN (SELECT warehouse, instock
                        FROM warehouses
                        WHERE stock_item= orders.item
                        AND instock >= orders.ordered );