How to update array of embedded collection field based on another collection embedded field value in MongoDB?

Question

I've searched in this forum for my below issue and I'm not able to find solution.

Inventory collction:

{
"_id" : ObjectId("555b1978af015394d1016374"),
"Billno" : "ABC1",
"Device_id" : "strsdedgfrtg12",
"item" : [ 
    {
        "item_id" : 232,
        "size" : "S",
        "qty" : 25
    }, 
    {
        "item_id" : 272,
        "size" : "M",
        "qty" : 5
    }
],
"category" : "clothing"

}

inventory_new collection:

---

{
"_id" : ObjectId("555b1978af015394d1016374"),
"Billno" : "ABC1",
"Device_id" : "strsdedgfrtg12",
"item" : [ 
    {
        "item_id" : 232,
        "size" : "S",
        "qty" : 25
    }, 
    {
        "item_id" : 272,
        "size" : "M",
        "qty" : 5000
    }
],
"category" : "clothing"

}

Now I've to update the inventory collection embedded array's item "qty" with inventory_new collections embedded item "qty" field value.. I've tried below code.. But I'm not succeed. Please advice.

db.inventory.find().forEach(function (doc1) {
var doc2 = db.inventory_copy.find({ Billno: doc1.Billno},{ Device_id: doc1.Device_id},{ item.item_id: doc1.item.item_id}, {item.qty: 1 });
if (doc2 != null) {
    doc1.item.qty = doc2.item.qty;
    db.inventory.save(doc1);
}

});

Thanks

Answer 1

Try the following update:

db.inventory.find().forEach(function (doc1) {
    var doc2 = db.inventory_copy.findOne(
        { 
            "Billno": doc1.Billno, 
            "Device_id": doc1.Device_id,
            "item.item_id": doc1.item.item_id
        }
    );
    if (doc2 != null) { 
        doc1.item = doc2.item;    
        db.inventory.save(doc1);
    }
});