MongoDB monthly count empty months

I need to get count for all my orders. What I've done so far is getting count for all months in which there are orders, I need to get all months and if there are no orders count should be 0. How can I achieve that.

database.collection('orders').aggregate([
{"$match": {"state": "finished"}},
{
"$project": {
"y": {
"$year": "$order_date"
},
"m": {
"$month": "$order_date"
}
}
},
{
"$group": {
"_id": {
"month": "$m",
"year": "$y"
},
count: {
"$sum": 1
}
}
}])


This produces following output:

[
{
"_id": {
"month": 9,
"year": 2017
},
"count": 2
},
{
"_id": {
"month": 8,
"year": 2017
},
"count": 4
},
{
"_id": {
"month": 11,
"year": 2017
},
"count": 4
},
{
"_id": {
"month": 10,
"year": 2017
},
"count": 3
},
{
"_id": {
"month": 4,
"year": 2017
},
"count": 2
},
{
"_id": {
"month": 6,
"year": 2017
},
"count": 1
},
{
"_id": {
"month": 3,
"year": 2017
},
"count": 9
},
{
"_id": {
"month": 7,
"year": 2017
},
"count": 2
},
{
"_id": {
"month": 2,
"year": 2017
},
"count": 3
},
{
"_id": {
"month": 12,
"year": 2017
},
"count": 3
},
{
"_id": {
"month": 1,
"year": 2017
},
"count": 2
}
]


There is no results for May so I need to get count 0 for May.

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