How to combine mongodb original output of query with some new fields? - mongodb

Question

Collection:
[
{
"name": "device1",
"type": "a",
"para": {
"number": 3
}
},
{
"name": "device2",
"type": "b",
"additional": "c",
"para": {
"number": 1
}
}
]
My query:
db.collection.aggregate([
{
"$addFields": {
"arrayofkeyvalue": {
"$objectToArray": "$$ROOT"
}
}
},
{
"$unwind": "$arrayofkeyvalue"
},
{
"$group": {
"_id": null,
"allkeys": {
"$addToSet": "$arrayofkeyvalue.k"
}
}
}
])
The output currently:
[
{
"_id": null,
"allkeys": [
"additional",
"_id",
"para",
"type",
"name"
]
}
]
Detail see Playground
What I want to do is add a new column which includes all of top key of the mongodb query output, exclude "para". And then combine it with the old collection to form a new json.
Is it possible?
The expected result:
{
"column": [{"prop": "name"}, {"prop": "type"}, {"prop": "additional"}],
"columnData": [
{
"name": "device1",
"type": "a",
"para": {
"number": 3
}
},
{
"name": "device2",
"type": "b",
"additional": "c",
"para": {
"number": 1
}
}
]
}

Answer 1

You have the right general idea in mind, here's how I would do it by utilizing operators like $filter, $map and $reduce to manipulate the objects structure.
I separated the aggregation into 3 parts for readability but you can just merge stage 2 and 3 if you wish.
db.collection.aggregate([
{
"$group": {
"_id": null,
columnData: {
$push: "$$ROOT"
},
"keys": {
"$push": {
$map: {
input: {
"$objectToArray": "$$ROOT"
},
as: "field",
in: "$$field.k"
}
}
}
}
},
{
"$addFields": {
unionedKeys: {
$filter: {
input: {
$reduce: {
input: "$keys",
initialValue: [],
in: {
"$setUnion": [
"$$this",
"$$value"
]
}
}
},
as: "item",
cond: {
$not: {
"$setIsSubset": [
[
"$$item"
],
[
"_id",
"para"
]
]
}
}
}
}
}
},
{
$project: {
_id: 0,
columnData: 1,
column: {
$map: {
input: "$unionedKeys",
as: "key",
in: {
prop: "$$key"
}
}
}
}
}
])
Mongo Playground