What is the correct way to use functions in MongoDB for sql joins-like query?

Question

MongoDB

There are two collections.
inventory:

> db.inventory.find().pretty()
{
        "_id" : 1,
        "sku" : "almonds",
        "description" : "product 1",
        "instock" : 120
}
{ "_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 }

And orders:

> db.orders.find().pretty()
{ "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 }
{ "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 }
{ "_id" : 3 }

I am running the following query:

db.orders.find().forEach(
  function (newOrders) {
    newOrders.item = db.inventory.findOne({"sku": newOrders.item});
    db.ordersRealoded.insert(newOrders);
  }
)

But after that an error is thrown:

2018-08-07T14:18:10.868+0300 E QUERY    [js] Error: error: {
        "ok" : 0,
        "errmsg" : "cannot compare to undefined",
        "code" : 2,
        "codeName" : "BadValue"
} :
[email protected]/mongo/shell/utils.js:25:13
[email protected]/mongo/shell/query.js:707:1
[email protected]/mongo/shell/query.js:113:28
[email protected]/mongo/shell/query.js:287:5
[email protected]/mongo/shell/collection.js:254:10
@(shell):3:18
[email protected]/mongo/shell/query.js:500:1
@(shell):1:1

What can be the problem? It is necessary that the new collection has an item field with all the values ​​from inventory.sku.
Now I'm trying to do this on a simple example in order to deal with nested queries. I would be glad if you leave links to some Russian-language doc or tutorial on this topic.
Thank you.