MongoDB replace sub doc Id, with the document - mongodb

Question

I am sure it was answered before but I can't find any proper explanation, maybe the aggregate methods I assumed are not relevant here.
I have a mongo collection containing an array which points to sub-docs
{
_id: '123',
name: 'my shop',
items: [
{
itemId: '234',
},
{
itemId: '345',
},
]
}
This is the collection of sub docs:
{
_id: '234',
name: 'apple',
amount: 13
},
{
_id: '345',
name: 'orange',
amount: 25
},
How can I replace in the upper document the id-reference to the actual document content.
Desired final result:
{
_id: '123',
name: 'my shop',
items: [
{
itemId: '234',
name: 'apple',
amount: 13
},
{
itemId: '345',
name: 'orange',
amount: 25
},
]
}

Answer 1

Let's say sample data for shop collection is:
{
_id: '123',
name: 'my shop',
items: [
{
itemId: '234',
},
{
itemId: '345',
},
]
}
And the sample data for item collection is:
{
_id: '234',
name: 'apple',
amount: 13
},
{
_id: '345',
name: 'orange',
amount: 25
}
Then the aggregation will be like this one:
db.shop.aggregate([{
$lookup: {
from: 'item',
localField: 'items.itemId',
foreignField: '_id',
as: 'items'
}
}])