MongoDB - How to make a query to get the average usage? - sql

Question

Here is how the documents in db look like.
/* 1 */
{
"_id" : 1,
"feat" : {
"processName": [
{
"value" : {
"value": "Process1"
}
}
],
"processUsage": [
{
"value" : {
"value": 23.21
}
}
]
}
}
/* 2 */
{
"_id" : 2,
"feat" : {
"processName": [
{
"value" : {
"value": "Process2"
}
}
],
"memoryUsage": [
{
"value" : {
"value": 2.411502e+05
}
}
]
}
}
/* 3 */
{
"_id" : 3,
"feat" : {
"processName": [
{
"value" : {
"value": "Process1"
}
}
],
"processUsage": [
{
"value" : {
"value": 67.42
}
}
]
}
}
/* 4 */
{
"_id" : 4,
"feat" : {
"processName": [
{
"value" : {
"value": "Process3"
}
}
],
"processUsage": [
{
"value" : {
"value": 39.97
}
}
]
}
}
/* 5 */
{
"_id" : 5,
"feat" : {
"processName": [
{
"value" : {
"value": "Process2"
}
}
],
"processUsage": [
{
"value" : {
"value": 21.05
}
}
]
}
}
Each process has entries with processUsage and memoryUsage. What I am interest in is the average processUsage. So, I'd like to ignore the entries with memoryUsage.
I tried $match + $group in an aggregate with $avg, but for each process I just got back as average 0.00000000.
Then I tried my luck with mapReduce using javascript, unfortunately it did not work out either.
Could someone just show me how to do that? By the way, I am using Robomongo 0.8.5
Edit:
The query looks like this:
db.database.aggregate([
{ $match : {"$feat.processUsage.value.value": {$gt : -1}
},
{
$group: {_id: "$feats.processName.value.value", average: {$avg:
"$feats.processUsage.value.value"}
}
])

Answer 1

You can use the following aggregate query:
db.test.aggregate(
[
{
$unwind : "$feat.processUsage"
},
{
$group: {
_id: "$feat.processName.value.value",
average: {$avg:"$feat.processUsage.value.value"}
}
}
]
)
Unwinding in the initial phase will let you filter documents that has processUsage key in document.
Result:
{ "_id" : [ "Process2" ], "average" : 21.05 }
{ "_id" : [ "Process3" ], "average" : 39.97 }
{ "_id" : [ "Process1" ], "average" : 45.315 }