Query the map size in MongoDB -

suppose have map of variable size in mongodb. how can documents have map size, example >2?

{map:{k1:v1, k2:v2, k3,v3}}  {map:{k1:v1, k2:v2}} 

you'd need pre-calculate such map size if want queries on them later consider creating new field "keycount" holds number of keys map subdocument. consider following demonstration:

populate test collection

db.test.insert([         {            "_id" : 1,           "map" : {             "k1" : "v1",             "k2" : "v2",             "k3" : "v3"         }     },     {                "_id" : 2,         "map" : {             "k1" : "v1",             "k2" : "v2"         }     }  ]); 

as current design, need mechanism count of keys inside map document. possible through map-reduce. following mapreduce operation populate separate collection new field "keycount" added:

var mr = db.runcommand({     "mapreduce": "test",     "map" : function() {         var obj = this;         obj['keycount'] = object.keys(this.map).length;         emit(this._id, obj);      },     "reduce" : function(key, stuff) { return null; },      "out": "my_collection" + "_keys" }) 

to documents have map size, example > 2, run query on resulting collection:

db[mr.result].find({ "value.keycount": { "$gt": 2 } }); 

map reduce output

/* 0 */ {     "result" : "my_collection_keys",     "timemillis" : 7,     "counts" : {         "input" : 2,         "emit" : 2,         "reduce" : 0,         "output" : 2     },     "ok" : 1 } 

query output

/* 0 */ {     "_id" : 1,     "value" : {         "_id" : 1,         "map" : {             "k1" : "v1",             "k2" : "v2",             "k3" : "v3"         },         "keycount" : 3     } }