$group inner array values without $unwind - mongodb

Question

I want to group objects in the array by same value for specified field and produce a count.
I have the following mongodb document (non-relevant fields are not present).
{
arrayField: [
{ fieldA: value1, ...otherFields },
{ fieldA: value2, ...otherFields },
{ fieldA: value2, ...otherFields }
],
...otherFields
}
The following is what I want.
{
arrayField: [
{ fieldA: value1, ...otherFields },
{ fieldA: value2, ...otherFields },
{ fieldA: value2, ...otherFields }
],
newArrayField: [
{ fieldA: value1, count: 1 },
{ fieldA: value2, count: 2 },
],
...otherFields
}
Here I grouped embedded documents by fieldA.
I know how to do it with unwind and 2 group stages the following way. (irrelevant stages are ommited)
Concrete example
// document structure
{
_id: ObjectId(...),
type: "test",
results: [
{ choice: "a" },
{ choice: "b" },
{ choice: "a" }
]
}
db.test.aggregate([
{ $match: {} },
{
$unwind: {
path: "$results",
preserveNullAndEmptyArrays: true
}
},
{
$group: {
_id: {
_id: "$_id",
type: "$type",
choice: "$results.choice",
},
count: { $sum: 1 }
}
},
{
$group: {
_id: {
_id: "$_id._id",
type: "$_id.type",
result: "$results.choice",
},
groupedResults: { $push: { count: "$count", choice: "$_id.choice" } }
}
}
])

Answer 1

You can use below aggregation
db.test.aggregate([
{ "$addFields": {
"newArrayField": {
"$map": {
"input": { "$setUnion": ["$arrayField.fieldA"] },
"as": "m",
"in": {
"fieldA": "$$m",
"count": {
"$size": {
"$filter": {
"input": "$arrayField",
"as": "d",
"cond": { "$eq": ["$$d.fieldA", "$$m"] }
}
}
}
}
}
}
}}
])

Answer 2

The below adds a new array field, which is generated by:
Using $setUnion to get unique set of array items, with inner $map to
extract only the choice field
Using $map on the unique set of items,
with inner $reduce on the original array, to sum all items where
choice matches
Pipeline:
db.test.aggregate([{
$addFields: {
newArrayField: {
$map: {
input: {
$setUnion: [{
$map: {
input: "$results",
in: { choice: "$$this.choice" }
}
}
]
},
as: "i",
in: {
choice: '$$i.choice',
count: {
$reduce: {
input: "$results",
initialValue: 0,
in: {
$sum: ["$$value", { $cond: [ { $eq: [ "$$this.choice", "$$i.choice" ] }, 1, 0 ] }]
}
}
}
}
}
}
}
}])
The $reduce will iterate over the results array n times, where n is the number of unique values of choice, so the performance will depend on that.