MongoDB aggregation: How can I get a count of columns based on a criteria - mongodb

Question

I have the following documents:
{ "col1": "camera", "fps": 1, "lat": 3 },
{ "col1": "camera", "fps": 3, "lat": 2 }
{ "col1": "camera", "foo": 9, "bar": 7 }
{ "col1": "camera", "bar": 8, "bar": 1 }
{ "col1": "camera", "check": 4, "lat": 3 }
How can I get the following:
{ "fps": 2, "lat": 3, "bar": 3, "check": 1, "foo": 1 }
Where each of these values are the number of occurrences (count) of each key (fps appears 2, foo, appears once, etc)

Answer 1

Aggregate
Here a solution (i named my collection cameras) if keys are known in advance :
db.cameras.aggregate([
{
$project: {
_id: 1,
fps: {
$ifNull: [ "$fps", 0 ]
},
lat: {
$ifNull: [ "$lat", 0 ]
},
bar: {
$ifNull: [ "$bar", 0 ]
},
foo: {
$ifNull: [ "$foo", 0 ]
},
check: {
$ifNull: [ "$check", 0 ]
}
}
},
{
$project: {
_id: 1,
fps: {
$cond: [ { $eq: [ "$fps", 0 ] } , 0, 1 ]
},
lat: {
$cond: [ { $eq: [ "$lat", 0 ] } , 0, 1 ]
},
bar: {
$cond: [ { $eq: [ "$bar", 0 ] } , 0, 1 ]
},
foo: {
$cond: [ { $eq: [ "$foo", 0 ] } , 0, 1 ]
},
check: {
$cond: [ { $eq: [ "$check", 0 ] } , 0, 1 ]
}
}
},
{
$group: {
_id: null,
fps: {
$sum: "$fps"
},
lat: {
$sum: "$lat"
},
bar: {
$sum: "$bar"
},
foo: {
$sum: "$foo"
},
check: {
$sum: "$check"
}
}
}
])
Results :
{
"result" : [
{
"_id" : null,
"fps" : 2,
"lat" : 3,
"bar" : 2,
"foo" : 1
}
],
"ok" : 1
}
MapReduce
Another solution if keys are unknown is mapReduce :
db.cameras.mapReduce(
function() {
var keys = Object.keys(this);
keys.forEach(function(key) {
emit(key, 1);
});
},
function(key, values) {
return Array.sum(values);
},
{
query: {},
out: {
inline: 1
}
}
)
Results :
{
"results" : [
{
"_id" : "_id",
"value" : 5
},
{
"_id" : "bar",
"value" : 2
},
{
"_id" : "check",
"value" : 1
},
{
"_id" : "col1",
"value" : 5
},
{
"_id" : "foo",
"value" : 1
},
{
"_id" : "fps",
"value" : 2
},
{
"_id" : "lat",
"value" : 3
}
],
"timeMillis" : 1,
"counts" : {
"input" : 5,
"emit" : 19,
"reduce" : 5,
"output" : 7
},
"ok" : 1,
}
But the result is not exactly in the same format.