$avg in mongodb aggregation - mongodb

Question

Document looks like this:
{
"_id" : ObjectId("361de42f1938e89b179dda42"),
"user_id" : "u1",
"evaluator_id" : "e1",
"candidate_id" : ObjectId("54f65356294160421ead3ca1"),
"OVERALL_SCORE" : 150,
"SCORES" : [
{ "NAME" : "asd", "OBTAINED_SCORE" : 30}, { "NAME" : "acd", "OBTAINED_SCORE" : 36}
]
}
Aggregation function:
db.coll.aggregate([ {$unwind:"$SCORES"}, {$group : { _id : { user_id : "$user_id", evaluator_id : "$evaluator_id"}, AVG_SCORE : { $avg : "$SCORES.OBTAINED_SCORE" }}} ])
Suppose if there are two documents with same "user_id" (say u1) and different "evaluator_id" (say e1 and e2).
For example:
1) Average will work like this ((30 + 20) / 2 = 25). This is working for me.
2) But for { evaluator_id : "e1" } document, score is 30 for { "NAME" : "asd" } and { evaluator_id : "e2" } document, score is 0 for { "NAME" : "asd" }. In this case, I want the AVG_SCORE to be 30 only (not (30 + 0) / 2 = 15).
Is it possible through aggregation??
Could any one help me out.

Answer 1

It's possible by placing a $match between the $unwind and $group aggregation pipelines to first filter the arrays which match the specified condition to include in the average computation and that is, score array where the obtained score is not equal to 0 "SCORES.OBTAINED_SCORE" : { $ne : 0 }
db.coll.aggregate([
{
$unwind: "$SCORES"
},
{
$match : {
"SCORES.OBTAINED_SCORE" : { $ne : 0 }
}
},
{
$group : {
_id : {
user_id : "$user_id",
evaluator_id : "$evaluator_id"
},
AVG_SCORE : {
$avg : "$SCORES.OBTAINED_SCORE"
}
}
}
])
For example, the aggregation result for this document:
{
"_id" : ObjectId("5500aaeaa7ef65c7460fa3d9"),
"user_id" : "u1",
"evaluator_id" : "e1",
"candidate_id" : ObjectId("54f65356294160421ead3ca1"),
"OVERALL_SCORE" : 150,
"SCORES" : [
{
"NAME" : "asd",
"OBTAINED_SCORE" : 0
},
{
"NAME" : "acd",
"OBTAINED_SCORE" : 36
}
]
}
will yield:
{
"result" : [
{
"_id" : {
"user_id" : "u1",
"evaluator_id" : "e1"
},
"AVG_SCORE" : 36
}
],
"ok" : 1
}