Can't access specific array elements in $group

mongos> db.test.find()

{ "_id" : ObjectId("548608660094d27682c03666"), "campaign_id" : 1, "channel1" : [ { "day" : 0, "delivered" : 10 }, { "day" : 1, "delivered" : 1 } ], "channel2" : [ { "day" : 0, "delivered" : 5 }, { "day" : 1, "delivered" : 6 } ] }

{ "_id" : ObjectId("548608a20094d27682c03667"), "campaign_id" : 2, "channel1" : [ { "day" : 0, "delivered" : 1 }, { "day" : 1, "delivered" : 1 } ], "channel2" : [ { "day" : 0, "delivered" : 5 }, { "day" : 1, "delivered" : 5 } ], "channel3" : [ { "day" : 0, "delivered" : 11 }, { "day" : 1, "delivered" : 1 } ] }

mongos> db.test.aggregate({$unwind : "$channel1"},{$group : {_id : {campaign_id : "$campaign_id", day : "$channel1.day"}, xxx: {$first : "$channel2"} }})

{ "_id" : { "campaign_id" : 2, "day" : 1 }, "xxx" : [ { "day" : 0, "delivered" : 5 }, { "day" : 1, "delivered" : 5 } ] }

{ "_id" : { "campaign_id" : 1, "day" : 1 }, "xxx" : [ { "day" : 0, "delivered" : 5 }, { "day" : 1, "delivered" : 6 } ] }

{ "_id" : { "campaign_id" : 2, "day" : 0 }, "xxx" : [ { "day" : 0, "delivered" : 5 }, { "day" : 1, "delivered" : 5 } ] }

{ "_id" : { "campaign_id" : 1, "day" : 0 }, "xxx" : [ { "day" : 0, "delivered" : 5 }, { "day" : 1, "delivered" : 6 } ] }

mongos> db.test.aggregate({$unwind : "$channel1"},{$group : {_id : {campaign_id : "$campaign_id", day : "$channel1.day"}, xxx: {$first : "$channel2.0.day"} }})

{ "_id" : { "campaign_id" : 2, "day" : 1 }, "xxx" : [ ] }

{ "_id" : { "campaign_id" : 1, "day" : 1 }, "xxx" : [ ] }

{ "_id" : { "campaign_id" : 2, "day" : 0 }, "xxx" : [ ] }

{ "_id" : { "campaign_id" : 1, "day" : 0 }, "xxx" : [ ] }

mongos> db.test.aggregate({$unwind : "$channel1"},{$group : {_id : {campaign_id : "$campaign_id", day : "$channel1.day"}, xxx: {$first : "$channel2.0"} }})

{ "_id" : { "campaign_id" : 2, "day" : 1 }, "xxx" : [ ] }

{ "_id" : { "campaign_id" : 1, "day" : 1 }, "xxx" : [ ] }

{ "_id" : { "campaign_id" : 2, "day" : 0 }, "xxx" : [ ] }

{ "_id" : { "campaign_id" : 1, "day" : 0 }, "xxx" : [ ] }

mongos> db.test.aggregate({$unwind : "$channel1"},{$group : {_id : {campaign_id : "$campaign_id", day : "$channel1.day"}, xxx: {$first : "$channel2[0]"} }})

{ "_id" : { "campaign_id" : 2, "day" : 1 }, "xxx" : null }

{ "_id" : { "campaign_id" : 1, "day" : 1 }, "xxx" : null }

{ "_id" : { "campaign_id" : 2, "day" : 0 }, "xxx" : null }

{ "_id" : { "campaign_id" : 1, "day" : 0 }, "xxx" : null }