MongoDB Aggregation Framework - Dynamic Field Rename - mongodb

Question

I am finding the MongoDB aggregation framework to be extremely powerful - it seems like a good option to flatten out an object. My schema uses a an array of sub objects in an array called materials. The number of materials is variable, but a specific field category will be unique across objects in the array. I would like to use the aggregation framework to flatten the structure and dynamically rename the fields based on the value of the category field. I could not find an easy way to accomplish this using a $project along with $cond. Is there a way?
The reason for the array of material objects is to allow simple searching:
e.g. { 'materials.name' : 'XYZ' } pulls back any document where "XYZ" is found.
E.g. of before and after document
{
"_id" : ObjectId("123456"),
"materials" : [
{
"name" : "XYZ",
"type" : "Red",
...
"category" : "A"
},
{
"name" : "ZYX",
"type" : "Blue",
...
"category" : "B"
}]
}
to
{
"material_A_name" : "XYZ",
"material_A_type" : "Red",
...
"material_B_name" : "ZYX",
"material_B_type" : "Blue",
...
}

Answer 1

There is a request for something like this in jira https://jira.mongodb.org/browse/SERVER-5947 - vote it up if you would like to have this feature.
Meanwhile, there is a work-around if you know up front what the possible values of the keys will be (i.e. all unique values of "category") and I have some sample code on it on my blog.

Answer 2

This would be useful from MongoDB v4.4,
$map to iterate loop of materials array
$map to iterate loop of name and type fields after converting to array using $objectToArray, concat your key fields requirement as per fields and value using $concat,
back to first $map convert returned result from second $map from array to object using $arrayToObject
$unwind deconstruct materials array
$group by null and merge materials object to one object
$replaceRoot to replace object in root
db.collection.aggregate([
{
$project: {
materials: {
$map: {
input: "$materials",
as: "m",
in: {
$arrayToObject: [
{
$map: {
input: {
$objectToArray: {
name: "$$m.name",
type: "$$m.type"
}
},
in: {
k: { $concat: ["material", "_", "$$m.category", "_", "$$this.k"] },
v: "$$this.v"
}
}
}
]
}
}
}
}
},
{ $unwind: "$materials" },
{
$group: {
_id: null,
materials: { $mergeObjects: "$materials" }
}
},
{ $replaceRoot: { newRoot: "$materials" } }
])
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