MongoDB Grouping and Project Custom Fields - mongodb

Question

I am stuck with MongoDB grouping and project custom fields. I have the following collection:
{
"DescId" : "1",
"Desc" : "Testing",
"ParentId" : "null",
"Order" : 1.0,
"Type" : "A",
"Parent" : null
}
{
"DescId" : "1.1",
"Desc" : "Testing Child 1",
"ParentId" : "1",
"Order" : 1.0,
"Type" : "B",
"Parent" : "Testing"
}
{
"DescId" : "1.2",
"Desc" : "Testing Child 2",
"ParentId" : "1",
"Order" : 2.0,
"Type" : "B",
"Parent" : "Testing"
}
I have done following grouping based on Type, DescId, Desc fields and projected DescId and Desc
db.getCollection("GenericData").aggregate(
[
{
"$group" : {
"_id" : {
"Type" : "$Type",
"DescId" : "$DescId",
"DescName" : "$Desc"
}
}
},
{
"$project" : {
"_id" : 0.0,
"Id" : "$_id.DescId",
"Name" : "$_id.DescName",
"Type" : "$_id.Type"
}
}
],
{
"allowDiskUse" : false
}
);
This is the output I am getting:
{
"Id" : "1.2",
"Name" : "Testing Child 2",
"Type" : "B"
}
{
"Id" : "1.1",
"Name" : "Testing Child 1",
"Type" : "B"
}
{
"Id" : "1",
"Name" : "Testing",
"Type" : "A"
}
Would it be possible to project fields based on Type field, like concatenating Type value with field name like following:
{
"A" + "Id" : "1",
"A" + "Name" : "Testing"
},
{
"B" + "Id" : "1.1",
"B" + "Name" : "Testing Child 1"
}
{
"B" + "Id" : "1.2",
"B" + "Name" : "Testing Child 2"
}

Answer 1

To rename your object's keys you have to use $objectToArray and $arrayToObject operators. First one can convert your $$ROOT object into an array of keys and values. Then you can apply $map to modify keys (using $concat) and $filter to exclude Type key. Then you can convert that array back to an object using $arrayToObject and promote that object to a root level using $replaceRoot. So you can add below stage to your aggregation pipeline:
db.GenericData.aggregate([
{
$replaceRoot: {
newRoot: {
$arrayToObject: {
$map: {
input: {
$filter: {
input: { $objectToArray: "$$ROOT" },
as: "kv",
cond: { $in: [ "$$kv.k", [ "Id", "Name" ] ] }
}
},
as: "kv",
in: {
k: { $concat: [ "$Type", "$$kv.k" ] },
v: "$$kv.v"
}
}
}
}
}
}
])
outputs:
{ "BId" : "1.2", "BName" : "Testing Child 2" }
{ "BId" : "1.1", "BName" : "Testing Child 1" }
{ "AId" : "1", "AName" : "Testing" }
EDIT:
If you'd like to explicitly specify which properties should be projected you can use $in operator inside $filter