fix error

temp_file/app/models/site.rb

@@ -82,7 +82,7 @@ class Site
       project_date_fields = Hash[*key_op.collect { |key, op| [key, {op => "$#{field}"}] }.flatten]
       group_id_fields = Hash[*key_op.collect { |key, op| [key, "$#{key}"] }.flatten]
       pipeline = [
-          {"$match"=> {"_id" => {"$gte" => limit_ele['_id']}}},
+          {"$match"=> {"_id" => {"$gte" => (limit_ele['_id'] rescue '')}}},
           {"$project" => {field => 1}.merge(project_date_fields)},
           {"$group" => {"_id" => group_id_fields,"count" => {"$sum" => 1}}},
           {"$sort" => {"_id"=>-1}}