Exclude 0 values from mongodb $avg but keeping other fields - mongodb

Question

I run some aggregation queries on MongoDB 3.2.
I would like to group documents by a field with an average on another numeric field.
I need the average to ignore the 0 values.
The problem is I can't entirely filter the document, cause there is another field I need for a count.
Let's illustrate :
This is the structure of my documents:
{"stringToGroupByOn":"foo", "valueToAvg":42, "valueToSum":21}
{"stringToGroupByOn":"foo", "valueToAvg":0, "valueToSum":13}
I can't just filter like this:
db.foobar.aggregate([
{
$match : { valueToAvg : { $gt : 0 } }
},
{
$group : {
_id : '$stringToGroupByOn',
avg : { $avg : '$valueToAvg' },
count : { $sum : '$valueToSum' }
}
}
])
Because I lose the value 13 for the count.
Do you think there is a way to do it in only one query ?

Answer 1

You can use $cond in projection to set null instead of 0, as null is not considered when using average.
db.avg.aggregate([
{$project:{
_id:1,
valueToSum:1,
stringToGroupByOn:1,
valueToAvg:{$cond:
{ if: { $eq: [ "$valueToAvg", 0 ] },
then: null,
else: "$valueToAvg" }}
}},
{
$group : {
_id : '$stringToGroupByOn',
avg : { $avg : '$valueToAvg' },
count : { $sum : '$valueToSum' }
}
}
output:
{
"_id" : "foo",
"avg" : 42.0,
"count" : 34.0
}