Count of MongoDB aggregation match results - mongodb

Question

I'm working with a MongoDB collection that has a lot of duplicate keys. I regularly do aggregation queries to find out what those duplicates are, so that I can dig in and find out what is and isn't different about them.
Unfortunately the database is huge and duplicates are often intentional. What I'd like to do is to find the count of keys that have duplicates, instead of printing a result with thousands of lines of output. Is this possible?
(Side Note: I do all of my querying through the shell, so solutions that don't require external tools or a lot of code would be preferred, but I understand that's not always possible.)
Example Records:
{ "_id" : 1, "type" : "example", "key" : "111111", "value" : "abc" }
{ "_id" : 2, "type" : "example", "key" : "222222", "value" : "def" }
{ "_id" : 3, "type" : "example", "key" : "222222", "value" : "ghi" }
{ "_id" : 4, "type" : "example", "key" : "333333", "value" : "jkl" }
{ "_id" : 5, "type" : "example", "key" : "333333", "value" : "mno" }
{ "_id" : 6, "type" : "example", "key" : "333333", "value" : "pqr" }
{ "_id" : 7, "type" : "example", "key" : "444444", "value" : "stu" }
{ "_id" : 8, "type" : "example", "key" : "444444", "value" : "vwx" }
{ "_id" : 9, "type" : "example", "key" : "444444", "value" : "yz1" }
{ "_id" : 10, "type" : "example", "key" : "444444", "value" : "234" }
Here is the query that I've been using to find duplicates based on key:
db.collection.aggregate([
{
$match: {
type: "example"
}
},
{
$group: {
_id: "$key",
count: {
$sum: 1
}
}
},
{
$match: {
count: {
$gt: 1
}
}
}
])
Which gives me an output of:
{
"_id": "222222",
"count": 2
},
{
"_id": "333333",
"count": 3
},
{
"_id": "444444",
"count": 4
}
The result I want to get instead:
3

Answer 1

You are almost there, just missing the last $count:
db.collection.aggregate([
{
$match: {
type: "example"
}
},
{
$group: {
_id: "$key",
count: {
$sum: 1
}
}
},
{
$match: {
count: {
$gt: 1
}
}
},
{
$count: "count"
}
])

Answer 2

Akrion's answer seems to be correct, but I can't test it because we're on an older version of MongoDB. A coworker gave me an alternative solution that works on 3.2 (not sure about other versions).
Adding .toArray() will convert the results to an array, and you can then get the size of the array using .length.
db.collection.aggregate([
{
$match: {
type: "example"
}
},
{
$group: {
_id: "$key",
count: {
$sum: 1
}
}
},
{
$match: {
count: {
$gt: 1
}
}
}
]).toArray().length