db.createCollection("counters")

{
  "_id":"productid",
  "sequence_value": 0
}

function (sequenceName){
   var sequenceDocument = db.counters.findAndModify(
      {
         query:{_id: sequenceName },
         update: {$inc:{sequence_value:1}},
         new:true
      });
   return sequenceDocument.sequence_value;
}

  model.id = Convert.ToInt32(_dbs.DataBase.Eval("getNextSequenceValue('productid')").AsBsonValue);