Add new column to multi nested array in mongo db - arrays

Question

"ParentType": {
"Food": [
{
"Name": "Burger",
"FoodId": "5e3abe145c1bfb31b4e335de",
"Price": 0,
"Quantity": 1,
"SubCategory": 0
}
],
"Inventory": [
{
"Name": "Small Popcorn",
"InventoryId": "5e3a64245c1bfb31b4e335b7",
"Price": 0,
"Quantity": 1,
"SubCategory": 0
}
]
}
I need to add UOM as new column only for Inventory array.I have used aggregate as below but collection is not getting updated.Pls help me with adding this new Column in mongodb
db.Concession.aggregate([
{
$addFields: { ParentType.Inventory.UOM: "null" }
}
])

Answer 1

add UOM to all eliment in Inventory
db.collection.update(
{},
{ $set: { 'ParentType.Inventory.$[].UOM':''} }
)

Answer 2

Option 1: ( Update/$set 3.6+ )
db.collection.update({},
{
$set: {
"ParentType.Inventory.$[].UOM": null
}
},
{
multi: true
})
Explained:
Use update() operation with positional operator $[] ( mongoDB 3.6+ )
Use option multi to update all documents in collection matching the filter criteria
Playground
Option 2: ( Update/aggregation 4.2+)
db.collection.update({},
[
{
$addFields: {
"ParentType.Inventory": {
"$map": {
input: "$ParentType.Inventory",
as: "i",
in: {
"$mergeObjects": [
"$$i",
{
UOM: null
}
]
}
}
}
}
}
],
{
multi: true
})
Explained:
Use update with aggregation pipeline ( mongoDB 4.2+ )
In aggregation use the addFileds with mergeObjects so you add the new fields to the array element.
Use multi:true to affect all documents in collection matching the filter criteria
Playground 2