We get undefined accessor error for the following pipeline.
db.b.explain().aggregate([ {$match: { master: 1} }, {$group: {"_id":"$status", "counter":{$sum:1}}}, {$group: { "_id": null, "pending":{$sum: {$cond:[{$lte:["$_id",99]},"$count",0]}}, "running":{$sum: {$cond:[{$gte:["$_id",100]},"$count",0]}} }} ]);