Hello i have simple collection:
{
_id: 1,
books: [
{ bookId: 55, c: 5},
{ bookId: 66, c: 6},
{ bookId: 77, c: 7},
]
}
How i can add new field by calulate other field? here i add field “Z” to current found object in nested array by it calculate field “C”
updateOne(
{
_id : 1,
'books.bookId' : 66
} ,
{
[
{ $addFields: { "books.$.z" : { "$sum" : ["$books.$.c", 1] } } }
]
}
Expected result:
{
_id: 1,
books: [
{ bookId: 55, c: 5},
{ bookId: 66, c: 6, z:7},
{ bookId: 77, c: 7},
]
}
I think there is a short entry (possibly using new $getField ?!), I think mango is still able to combine ‘position operator $’ + (‘varible operator reference by prefix $’ or ‘combine with $getField’) how i try in my sample
Use the aggregation pipeline in the update method to leverage the operators $map, $mergeObjects and $cond to achieve the desired results:
.updateOne(
// The query will find docs where *at least* one bookId in the
// books array equals 66 -- but remember it does not pinpoint
// the location in the array! We will tackle that in the update
// pipeline to follow...
{ _id: 1, 'books.bookId': 66 },
[
{ $set: {
books: {
$map: {
input: '$books',
in: {
$mergeObjects: [
'$$this',
{
$cond: [
{ $eq: ['$$this.bookId', 66] }, // IF books.bookId == 66
{ z: { $sum: ['$$this.c', 1] } }, // THEN merge a new field 'z' with calced value
null // ELSE merge null (a noop)
]
}
]
}
}
}
} }
]
)