MongoDB double $group aggregation - mongodb

Question

I have several documents that looks like this:
{
"_id" : ObjectId("50b59cd75bed76f46522c34e"),
"player_id" : 0,
"league_id" : 2,
"results" : [
{ "discipline" : "football",
"score" : 25.15
},
{
"discipline" : "basketball",
"score" : 21.24
},
{
"discipline" : "cycling",
"score" : 68.19
},]
}
I try to aggregate this data. First unwind results array, then leave only "football" and "cycling", next count average result. This part I did, and it is working.
My code:
db.grades.aggregate(
{$unwind:"$results"},
{$match: {$or: [{"results.discipline":"football"},{"results.discipline":"cycling"} ]}},
{$group:{_id:{player_id:"$player_id",league_id:"$league_id"}, 'average':{$avg:"$results.score"}}},
)
Then I try to aggregate by league_id, it means, average players results in specific leagues, add to code above:
{$group:{_id:"$_id.league_id",aver_league:{$avg:$average}}}
And now code looks like:
db.grades.aggregate(
{$unwind:"$results"},
{$match: {$or: [{"results.discipline":"football"},{"results.discipline":"cycling"} ]}},
{$group:{_id:{player_id:"$player_id",league_id:"$league_id"}, 'average':{$avg:"$results.score"}}},
{$group:{_id:"$_id.league_id",aver_league:{$avg:$average}}}
)
Console displays: JavaScript execution failed: ReferenceError: $average is not defined. What is wrong? Where did I make a mistake? Is it possible to aggregate by _id.league_id?

Answer 1

Try this pipeline:
[
{$unwind:"$results"},
{$match: {"results.discipline":{$in:["football", "basketball"]}}},
{$group{_id:{player_id:"$player_id",league_id:"$league_id"}, 'average':{$avg:"$results.score"}}}
]
it works for me with your doc:
{
"result" : [
{
"_id" : {
"player_id" : 0,
"league_id" : 2
},
"average" : 23.195
}
],
"ok" : 1
}
UPD. If you want to group again, by league_id:
[{$unwind:"$results"},
{$match: {"results.discipline":{$in:["football", "basketball"]}}},
{$group:{_id:{player_id:"$player_id",league_id:"$league_id"}, 'average':{$avg:"$results.score"} }},
{$group:{_id:"$_id.league_id", 'average':{$avg:"$average"} }} ]
{ "result" : [ { "_id" : 2, "average" : 23.195 } ], "ok" : 1 }