I have a MongoDB shell command which works fine. The command is:

db.sessions.aggregate({$unwind: "$Sessions List"}, {$match: { "Sessions List.Parent Session Name":"Reader Session"}}, {$group:{_id:"$Sessions List.Parent Session Name", avg_val:{$avg:"$Sessions List.Length"}}})

result is: "result" : [

{ "_id" : "Reader Session", "avg_val" : 8.846666666666666 }

],
"ok" : 1

but when trying to run it through Mongoid (using latest available Mongoid, Origin, Moped) it falls with NoMethodError (undefined method `bson_load' for nil:NilClass) exception.

Ruby code is the following:
Session.collection.aggregate({
'$unwind' => '$Sessions List',
'$match' =>

{ 'Sessions List.Parent Session Name' => 'Reader Session'}

,
'$group' => {
'_id' => '$Sessions List.Parent Session Name',
'avg_val' =>

{ '$avg' => '$Sessions List.Length' }

}});