Change element name from the result set of Mongo DB Query - mongodb

Question

I have collection like below named as "FormData",
{
"_id": ObjectId("5e3c27bf1ef77236945ef07b"),
"eed12747-0923-4290-b09c-5a05107f5609": "20200206",
"bd637691-782d-4cfd-8624-feeedfe11b3e": "20200206_1#mail.com"
}
I have another collection named as "Form" which will have Title of Fields,
{
"_id": ObjectId("5e3c27bf1ef77236945ef07b"),
"Fields":[
{
"FieldID": "eed12747-0923-4290-b09c-5a05107f5609",
"Title": "Phone"
},
{
"FieldID": "bd637691-782d-4cfd-8624-feeedfe11b3e",
"Title": "Email"
}]
}
Now I have to map element name with Form field title and I need result like below,
{
"_id": ObjectId("5e3c27bf1ef77236945ef07b"),
"Phone": "20200206",
"Email": "20200206_1#mail.com"
}
Please help me to solve this.
Thanks in advance!

Answer 1

You can:
$objectToArray to convert the $$ROOT document into an array of k-v pairs for future lookups
use a sub-pipeline in $lookup to find the value by the uuid
use $mergeObject to combine the original values(i.e. "20200206"...) with the new field name looked up (i.e. "Phone"...)
wrangle the result back into original form using $arrayToObject and $replaceRoot
db.FormData.aggregate([
{
$match: {
"_id": ObjectId("5e3c27bf1ef77236945ef07b")
}
},
{
$project: {
kv: {
"$objectToArray": "$$ROOT"
}
}
},
{
$unwind: "$kv"
},
{
"$lookup": {
"from": "Form",
"let": {
uuid: "$kv.k"
},
"pipeline": [
{
$match: {
"_id": ObjectId("5e3c27bf1ef77236945ef07b")
}
},
{
"$unwind": "$Fields"
},
{
$match: {
$expr: {
$eq: [
"$$uuid",
"$Fields.FieldID"
]
}
}
},
{
$project: {
_id: false,
k: "$Fields.Title"
}
}
],
"as": "formLookup"
}
},
{
$unwind: "$formLookup"
},
{
$project: {
kv: {
"$mergeObjects": [
"$kv",
"$formLookup"
]
}
}
},
{
$group: {
_id: "$_id",
kv: {
$push: "$kv"
}
}
},
{
"$project": {
newDoc: {
"$arrayToObject": "$kv"
}
}
},
{
"$replaceRoot": {
"newRoot": {
"$mergeObjects": [
{
"_id": "$_id"
},
"$newDoc"
]
}
}
}
])
Mongo Playground

Answer 2

Another option is to start from Form collection and avoid $unwind:
$match and $lookup to get all needed data into one document
$objectToArray to get known keys for FormData
Match the items using $indexOfArray and $arrayElemAt and merge them using $mergeObjects. Then use arrayToObject to format the response
db.Form.aggregate([
{$match: {_id: ObjectId("5e3c27bf1ef77236945ef07b")}},
{$lookup: {
from: "FormData",
localField: "_id",
foreignField: "_id",
as: "formLookup",
pipeline: [{$project: {_id: 0}}]
}},
{$set: {formLookup: {$objectToArray: {$first: "$formLookup"}}}},
{$replaceRoot: {
newRoot: {
$mergeObjects: [
{$arrayToObject: {
$map: {
input: "$formLookup",
in: {$mergeObjects: [
{v: "$$this.v"},
{k: {$getField: {
input: {$arrayElemAt: [
"$Fields",
{$indexOfArray: ["$Fields.FieldID", "$$this.k"]}
]},
field: "Title"
}}}
]}
}
}},
{_id: "$_id"}
]
}
}}
])
See how it works on the playground example