Mongodb group by values and count the number of occurence - mongodb

Question

I am trying to count how many times does a particular value occur in a collection.
{
_id:1,
field1: value,
field2: A,
}
{
_id:2,
field1: value,
field2: A,
}
{
_id:3,
field1: value,
field2: C,
}
{
_id:4,
field1: value,
field2: B,
}
what I want is to count how many times A occurs, B occurs and C occurs and return the count.
The output I want
{
A: 2,
B: 1,
C: 1,
}

Answer 1

You can use $facet in an aggregate pipeline like this:
$facet create "three ways" where in each one filter the values by desired key (A, B or C).
Then in a $project stage you can get the $size of the matched values.
db.collection.aggregate([
{
"$facet": {
"first": [
{
"$match": {
"field2": "A"
}
}
],
"second": [
{
"$match": {
"field2": "B"
}
}
],
"third": [
{
"$match": {
"field2": "C"
}
}
]
}
},
{
"$project": {
"A": {
"$size": "$first"
},
"B": {
"$size": "$second"
},
"C": {
"$size": "$third"
}
}
}
])
Example here

Answer 2

This is typical use case for $group stage in Aggregation Pipeline. You can do it like this:
$group - to group all the documents by field2
$sum - to count the number of documents for each value of field2
db.collection.aggregate([
{
"$group": {
"_id": "$field2",
"count": {
"$sum": 1
}
}
}
])
Working example

Answer 3

Leverage the $arrayToObject operator and a final $replaceWith pipeline to get the desired result. You would need to run the following aggregate pipeline:
db.collection.aggregate([
{ $group: {
_id: { $toUpper: '$field2' },
count: { $sum: 1 }
} },
{ $group: {
_id: null,
counts: {
$push: { k: '$_id', v: '$count' }
}
} },
{ $replaceWith: { $arrayToObject: '$counts' } }
])
Mongo Playground