Get the $size (length) of a nested array and calculate the difference to a stored value on the parent object - using aggregate - mongodb

Question

Let's consider that I have the following documents (ignoring the _id):
[
{
"Id": "Store1",
"Info": {
"Location": "Store1 Street",
"PhoneNumber": 111
},
"MaxItemsPerShelf": 3,
"Shelf": [
{
"Id": "Shelf1",
"Items": [
{
"Id": "Item1",
"Name": "bananas"
},
{
"Id": "Item2",
"Name": "apples"
},
{
"Id": "Item3",
"Name": "oranges"
}
]
},
{
"Id": "Shelf2",
"Items": [
{
"Id": "Item4",
"Name": "cookies"
},
{
"Id": "Item5",
"Name": "chocolate"
}
]
},
{
"Id": "Shelf3",
"Items": []
}
]
},
{
"Id": "Store3",
"Info": {
"Location": "Store2 Street",
"PhoneNumber": 222
},
"MaxItemsPerShelf": 2,
"Shelf": [
{
"Id": "Shelf4",
"Items": [
{
"Id": "Item6",
"Name": "champoo"
},
{
"Id": "Item7",
"Name": "toothpaste"
}
]
},
{
"Id": "Shelf5",
"Items": [
{
"Id": "Item8",
"Name": "chicken"
}
]
}
]
}
]
Given a specific Shelf.Id I want to get the following result ( Shelf.Id = "Shelf2"):
[{
"Info": {
"Location": "Store1 Street",
"PhoneNumber": 111
},
"ItemsNumber": 2,
"ItemsRemaining": 1
}]
Therefore:
ItemsNumberis the $size of Shelf
and
ItemsRemainingis equal to MaxItemsPerShelf $size of Shelf
also I want to copy the value of the Info to the aggregate output.
How can I accomplish this with aggregate? On my efforts I couldn't pass through an iterator that gets the $size of $Shelf.Items

Answer 1

You can use below aggregation
db.collection.aggregate([
{ "$match": { "Shelf.Id": "Shelf2" }},
{ "$replaceRoot": {
"newRoot": {
"$let": {
"vars": {
"shelf": {
"$filter": {
"input": {
"$map": {
"input": "$Shelf",
"in": {
"Id": "$$this.Id",
"count": { "$size": "$$this.Items" }
}
}
},
"as": "ss",
"cond": { "$eq": ["$$ss.Id", "Shelf2"] }
}
}
},
"in": {
"Info": "$Info",
"ItemsNumber": { "$arrayElemAt": ["$$shelf.count", 0] },
"ItemsRemaining": {
"$subtract": [
"$MaxItemsPerShelf",
{ "$ifNull": [
{ "$arrayElemAt": ["$$shelf.count", 0] },
0
]}
]
}
}
}
}
}}
])