Mongo Find (Un)matching documents in 2 collections

matching.js

use db1 

db.createCollection("scrapped");

db.scrapped.insert( 
    [
        { sku: "12345" },
        { sku: "23456" },
        { sku: "34567" },
        { sku: "45678" },
        { sku: "67890" }
    ]
);

db.createCollection("inventory");

db.inventory.insert( 
    [
        { code: "1234509" },
        { code: "2345698" },
        { code: "3456787" },
        { code: "4567876" },
        { code: "5678965" }
    ]
);


db.inventory.aggregate(
[ 
    { 
        $project : 
        { 
            code: 1,
            sku : { $substr : [ "$code", 0, 5 ] }
        }
    },
    {
        $lookup:
        {
            from: "scrapped",
            localField: "sku",
            foreignField: "sku",
            as: "matching"
        }
    },
    {
        $match: 
        {
            matching : { $size : 0 }
        }
    }
]
);