MongoDB aggregate reduce mergeObjects field name from value - mongodb

Question

I have an array in a Mongo document that has a number of properties. I want to produce a document that has a single object that represents the array.
{
scores_array: [
{ name: "One", score: 40, key: "abc" },
{ name: "Two", score: 50, key: "def" },
{ name: "Three", score: 40, key: "abc" },
{ name: "Four", score: 40, key: "ghi" },
}
}
What I want is to be able to query for documents that have a particular value for two of the properties in the array, for example, { name: "One", score: 40 }.
I can use $in to find documents that have the matching key or value, and I could redact them to filter them out but I need to reference them again later in the same pipeline, so I'm trying to find a way to filter without removing anything from the documents.
My plan is to use $reduce to produce a new property that looks like this:
scores_summary: {
One: 40,
Two: 50,
Three: 40,
Four: 40,
}
I'm having trouble getting the $reduce syntax right. Here's what I've got:
db.test.aggregate([
{
$match: {
scores_array: {$exists: true}
}
},
{
$project: {
scores_summary: {
$reduce: {
input: "$scores_array",
initialValue: {},
"in": {
"$mergeObjects": [
"$$value",
{
"$$this.name":"$$this.score"
}
]
}
}
}
}
},
]);
This gives me "Unrecognized expression $$this.name". If I replace that with a constant, I get an output that looks right, except for that property.
Is there a reason I can't use $$this.name as the property name in the mergeObjects in a reduce? Hoping I'm just missing something.
(I'm using Mongo 3.4 currently which means I can't just fall back to $expr).
Thanks

Answer 1

You can use $map and $arrayToObject
db.collection.aggregate([
{
$match: {
scores_array: {
$exists: true
}
}
},
{
$project: {
scores_summary: {
"$arrayToObject": {
$map: {
input: "$scores_array",
"in": {
k: "$$this.name",
v: "$$this.score"
}
}
}
}
}
}
])
Working Mongo playground