https://www.mongodb.com/community/forums/t/count-values-in-array-of-objects/13035/5
db.collection.aggregate([
{
"$project": {
"_id": 0,
"up": {
"$size": {
"$filter": {
"input": "$votes",
"as": "vote",
"cond": {
"$eq": [
"$$vote.action",
"up"
]
}
}
}
},
"down": {
"$size": {
"$filter": {
"input": "$votes",
"as": "vote",
"cond": {
"$eq": [
"$$vote.action",
"down"
]
}
}
}
}
}
}
])