mongodb - $Where gives error -

i have collection containing data:

{   "_id" : objectid("51dfb7abe4b02f15ee93a7c7"),   "date_created" : "2013-7-12  13:25:5",   "referrer_id" : 13,   "role_name" : "physician",   "status_id" : "1",  } 

i sending query:

cmd {     "mapreduce" : "doctor" ,     "map" : "function map(){emit(this._id,this);}" ,     "reduce" : "function reduce(key,values){return values;}" ,     "verbose" : true ,     "out" : { "merge" : "map_reduce"} ,     "query" : { "$where" : "this.demographics.first_name=='makdoctest'"} } 

i getting error as:

"errmsg" : "exception: count failed in dbdirectclient: 10071 error on invocation of $where function:\njs error: typeerror: this.demographics has no properties nofile_a:0" 

as sammaye says in comment:

it means somewhere in 1 of documents demographics null or not exist, need null check first, more importantly why dong in $where?

i go further that, , wouldn't use map/reduce mechanism here. slow, can't use indexes , can not run in parallel others.

you much better off using aggregation framework can like:

db.doctor.aggregate( [     { $match: { "demographics.first_name" : 'makdoctest' } },     { $group: … 

you didn't specify final goal here, once can update answer.