MongoDB: Updating a document with existing fields using aggregation - mongodb

Question

I am new to mongodb, I am learning from some Udemy courses and I want to know how I can update a document existing field without overwriting it.
I have the following collection with these documents:
enter image description here
I want to add new warehouses in the "item":"drafts" within the stock field.
What I am trying is:
enter image description here
And giving the output it seems that is working, but when I do again db.matrices.find(), what I get is the exactly same output that in the first image.
How can I update it? I have tried also the update method, but does not do what I want to do.
Thanks!
PD: I am using linux mint, with mongo v5.0.3, and mongosh v1.1.1

Answer 1

You are using the aggregate pipeline, this does not update the document in the DB, it just retrieves the result. starting in Mongo version 4.2+ you can now use an aggregation pipeline ( with some limitations ) to update a document, like so:
db.collection.updateOne({
item: "drafts"
},
[
{
$set: {
stock: {
$concatArrays: [
"$stock",
[
{
"warehouse": "A",
qty: 20
}
]
]
}
}
}
])
Mongo Playground
I will just say that this specific update is very simple, there is no need to use an aggregation pipeline for it. a simple use of the $push operator in the update field will suffice in a "normal" update:
db.collection.updateOne({
item: "drafts"
},
{
$push: {
stock: {
"warehouse": "A",
qty: 20
}
}
})
Mongo Playground