Why would I get a "-Infinity" result for $avg in an aggregate query? - mongodb

Question

My aggregate query for retrieving "average percents per month" returns a -Infinity average for some months. What would cause this?
The relative properties in mycollection are mydate and mynumericfield, which stores a percentage value as a double.
db.mycollection.aggregate(
[
{
$match: {
mydate: {
$gte: new Date(Date.UTC(2014, 8, 1)),
$lte: new Date(Date.UTC(2014, 12, 1)),
}
}
},
{
$group : {
_id : { month: { $month: "$mydate" }, year: { $year: "$mydate" } },
average: { $avg: "$mynumericfield" },
count: { $sum: 1 }
}
}
]
)
Here's a sample of the result:
/* 1 */
{
"result" : [
{
"_id" : {
"month" : 9,
"year" : 2014
},
"average" : 84.2586640583996598,
"count" : 20959.0000000000000000
},
{
"_id" : {
"month" : 11,
"year" : 2014
},
"average" : 96.9326915103888638,
"count" : 20743.0000000000000000
},
{
"_id" : {
"month" : 10,
"year" : 2014
},
"average" : -Infinity,
"count" : 20939.0000000000000000
},
{
"_id" : {
"month" : 12,
"year" : 2014
},
"average" : -Infinity,
"count" : 20913.0000000000000000
}
],
"ok" : 1.0000000000000000
}

Answer 1

I've managed to somehow reproduce the -Infinity problem but on a smaller scale.
Let's take sample collection nums having only 4 documents in it:
{ "_id" : 0, "grp" : 1, "mynum" : -Infinity }
{ "_id" : 1, "grp" : 1, "mynum" : 5 }
{ "_id" : 3, "grp" : 2, "mynum" : 8 }
{ "_id" : 4, "grp" : 2, "mynum" : 89 }
Performing simple aggregation on this collection like:
> db.nums.aggregate([{$group:{"_id":"$grp", "average" : {$avg : "$mynum"}}}])
gives the following result:
{ "_id" : 2, "average" : 48.5 }
{ "_id" : 1, "average" : -Infinity }
which is identical in effects to what you have experienced.
Please try to find out whether in your collection there is a document which has mynumericfield with value -Infinity - maybe your situation is similar to the reproduced one:
> db.mycollection.find({mynumericfield : -Infinity})
I hope it might help you some way.