I want to $addField for every document in a subArray with the name as arrayName_index (don't know how long the array is).
For example: I have a collection for fun Activities that looks like:
{
_id: "1",
activity: movie,
attendees: [
{firsName: "personA", lastName: "lasty"},
{firsName: "personB", lastName: "namey"},
{firsName: "personC", lastName: "blabla"}
]},
{
_id: "2",
activity: bowling,
attendees: [{firsName: "personA", lastName: "lasty"}]
}
I want a mongodb aggregation to create an output as:
{
_id: "1",
activity: movie,
attendees: [
{firsName: "personA", lastName: "lasty"},
{firsName: "personB", lastName: "namey"},
{firsName: "personC", lastName: "blabla"}
],
attendees_1: {firsName: "personA", lastName: "lasty"},
attendees_2: {firsName: "personB", lastName: "namey"},
attendees_3: {firsName: "personC", lastName: "blabla"},
},
{
_id: "2",
activity: bowling,
attendees: [{firsName: "personA", lastName: "lasty"}],
attendees_1: {firsName: "personA", lastName: "lasty"}
}
A bit long and complex query.
Concept
1.0 $replaceRoot - Replace the input document to specific document
1.1 $mergeObjects - Merge current document ($$ROOT) with the object generated in 1.2.
1.2 $arrayToObject - Convert array to single document from 1.3.
1.3 $reduce - Iterate each element from the array and transform it to a single value.
1.4 $concatArrays - Merge array.
1.4.1 Create document with
1.4.1.1 k (key) with prefix "attendees_" and get the index of current iterate document ($indexOfArray) + 1.
1.4.1.2 v (value) with current iterate document.
db.collection.aggregate([
{
"$replaceRoot": {
"newRoot": {
"$mergeObjects": [
"$$ROOT",
{
"$arrayToObject": {
"$reduce": {
"input": "$attendees",
"initialValue": [],
"in": {
"$concatArrays": [
"$$value",
[
{
k: {
"$concat": [
"attendees_",
{
"$toString": {
"$add": [
{
"$indexOfArray": [
"$attendees",
"$$this"
]
},
1
]
}
}
]
},
v: "$$this"
}
]
]
}
}
}
}
]
}
}
}
])
Maybe slightly better since this avoids array searches, but it's largely the same as Yong Shun's answer.
db.collection.aggregate([
{
"$replaceRoot": {
"newRoot": {
"$mergeObjects": [
"$$ROOT",
{
"$arrayToObject": {
"$reduce": {
"input": { "$range": [ 0, { "$size": "$attendees" } ] },
"initialValue": [],
"in": {
"$concatArrays": [
"$$value",
[
{
k: {
"$concat": [
"attendees_",
{ "$toString": { "$add": [ 1, "$$this" ] } }
]
},
v: { "$arrayElemAt": [ "$attendees", "$$this" ] }
}
]
]
}
}
}
}
]
}
}
}
])
Try it on mongoplayground.net.