Match and Average in mongo keep producing null - mongodb

Question

I'm using the console to perform an aggregation, using $match to check that a nested field exists, and then pushing to the group and $avg operator. However the match works, just fine on the same variable and the code for count works too, but when it comes to the average I return null every time.
I'm looking in an array with .0 for example for the first element and then looking in a field for that element. It's very perplexing and difficult to debug. Are there any suggestions? Distinct shows that the values I look at are all numeric afaik. Are the any suggestions for how to debug this?
db.b.aggregate([ {$match: {"x.x.x.0.x": {$exists: true} } }, {$group: {_id: null, myAvg: { $avg: "$x.x.x.0.x"}}}])
Results in:
{ "_id" : null, "myAvg" : null }

Answer 1

This appears to be a limitation of the aggregation framework with respect to where you can actually use the "array.n" notation to access the nth element of an array.
More precisely, given the following sample document:
db.test.insertOne({
"a" : [
{
"x" : 1.0
}
]
})
...you can do the following to retrieve all documents where the first element of the "a" array matches 1:
db.test.aggregate({
$match: {
"a.0.x": 1
}
})
However, you cannot run the following:
db.test.aggregate({
$project: {
"a0x": "$a.0.x"
}
})
Well, you can but it will return an empty array like this which is a little surprising indeed:
{
"_id" : ...,
"a0x" : []
}
However, there is a special operator $arrayElemAt to access the nth element in this case like so:
db.test.aggregate({
$project: {
"a0x": { $arrayElemAt: [ "$a.x", 0 ] },
}
})
Kindly note that this will return the nth element only - so not nested inside an array anymore:
{
"a0x" : 1.0
}
So what you probably want to do is this:
db.b.aggregate({
$group: {
_id: null,
myAvg: {
$avg: {
$arrayElemAt: [ "$x.x.x.x", 0 ]
}
}
}
})