mongodb无法取到数据，什么问题？

#include "mongo.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main ( int argc, char *argv[] )
{
   mongo conn[1];
   mongo_cursor* cursor;
   int status = mongo_client( conn, "127.0.0.1", 27017 );
   if( status != MONGO_OK ) {
      switch ( conn->err ) {
         case MONGO_CONN_NO_SOCKET:  printf( "no socket\n" ); return 1;
         case MONGO_CONN_FAIL:       printf( "connection failed\n" ); return 1;
         case MONGO_CONN_NOT_MASTER: printf( "not master\n" ); return 1;
      }
   }
   printf( "connection done\n" );
   
   bson query[1];
   bson_init(query);
   bson_append_string(query,"md5","51401d76302c4b1235d5e84d");
   bson_finish(query);
   
   bson fields[1];
   bson_init(fields);
   /*bson_append_string(fields,"name1","1");*/
   /*bson_append_int(fields,"age1",1);*/
   bson_finish(fields);
   
   cursor = mongo_find(conn,"mydb.fetch",query,fields,0,0,0);
   
   bson_destroy(query);
   bson_destroy(fields);
   
   while(mongo_cursor_next(cursor) == MONGO_OK)
   {
      bson_print(mongo_cursor_bson( cursor ));
   }

mongo_find 无法查询到数据放入到fields中请哪位朋友帮忙看看，谢谢了！