How to aggregate list in MongoDB? - mongodb

Question

I have a set of containers each with a set of items. The MongoDB document for a container looks like below. How do I list all the items in all the containers with the container name in each item?
{
"Container": 28392
...
"Items": [
{
"ItemName": "Foo",
...
},
{
"ItemName": "Bar",
...
}
]
}
Expected output:
[
{
"ItemName": "Foo",
"Container": 28392
...
},
{
"ItemName": "Bar",
"Container": 28392
...
},
{
"ItemName": "Baz",
"Container": 52892
...
}
]
Would this be possible with some kind of an unwind? If so how would I aggregate this?

Answer 1

After unwind you can use project stage to do what you want:
db.collection.aggregate([
{
$unwind: "$Items"
},
{
$project: {
Container: "$Container",
ItemName: "$Items.ItemName"
}
}
])
Playground:
https://mongoplayground.net/p/H-aWx07pCDt
Or if your mongodb version is equal or greater than 4.2, you can use replaceWith with mergeObjects, to get all fields in items array.
db.collection.aggregate([
{
$unwind: "$Items"
},
{
$replaceWith: {
$mergeObjects: [
{
Container: "$Container"
},
"$Items"
]
}
}
])
Playground:
https://mongoplayground.net/p/E5KqFlNq1CL
Input:
[
{
"Container": 28392,
"Items": [
{
"ItemName": "Foo",
"ItemDesc": "Foo desc"
},
{
"ItemName": "Bar",
"ItemDesc": "Bar desc"
}
]
}
]
Output:
[
{
"Container": 28392,
"ItemDesc": "Foo desc",
"ItemName": "Foo"
},
{
"Container": 28392,
"ItemDesc": "Bar desc",
"ItemName": "Bar"
}
]

Answer 2

Are you looking for a query like this?
db.collection.aggregate([
{
"$unwind": "$Items"
},
{
"$group": {
"_id": "$Container",
"All Items": {
"$push": {
"ItemName": "$Items.ItemName",
"Container": "$Container"
}
}
}
},
{
"$project": {
"_id": 0
}
}
])
Here's a link to a demo of the query - https://mongoplayground.net/p/QDxr-2VUa_f