diff --git a/sample/lua/userdb.lua b/sample/lua/userdb.lua
new file mode 100644
index 0000000..fed86f6
--- /dev/null
+++ b/sample/lua/userdb.lua
@@ -0,0 +1,73 @@
+#! /usr/bin/env lua
+--
+-- userdb.lua
+-- Copyright (C) 2024 Shewer Lu <shewer@gmail.com>
+--
+-- Distributed under terms of the MIT license.
+--
+--[[
+example:
+local userdb = require 'userdb'
+local ldb=userdb.LevelDb('ecdict')
+ldb:open()
+for k,v in ldb:query('a'):iter() do print(k,v) end
+
+--]]
+local db_pool_ = {}
+local vars_get= {
+  _loaded=true,
+  read_only=true,
+  disabled=true,
+  name=true,
+  file_name=true,
+  }
+local vars_set= {}
+local userdb_mt = {}
+function userdb_mt.__newindex(tab,key,value)
+  local db = db_pool_[tab._db_key]
+  if not db then
+    db = UserDb(tab._db_name, tab._db_class)
+    db_pool_[tab._db_key] = db
+  end
+  if db and vars_set[key] then
+    db[key]= value
+  end
+end
+  
+function userdb_mt.__index(tab,key)
+  local db = db_pool_[tab._db_key]
+  if not db then
+    db = UserDb(tab._db_name, tab._db_class)
+    db_pool_[tab._db_key] = db
+  end
+
+  if db and vars_get[key] then
+    return db[key]
+  else
+    return function (tab, ...)
+      return db[key](db,...)
+    end
+  end
+end
+
+local userdb= {}
+
+function userdb.UserDb(db_name, db_class)
+  local db_key = db_name .. "." .. db_class
+  local db = {
+    _db_key = db_key,
+    _db_name= db_name,
+    _db_class = db_class,
+  }
+  db_pool_[db_key] = UserDb(db_name, db_class)
+  return setmetatable(db , userdb_mt)
+end
+
+function userdb.LevelDb(db_name)
+  return userdb.UserDb(db_name, "userdb")
+end
+function userdb.TableDb(db_name)
+  return userdb.UserDb(db_name, "plain_userdb")
+end
+  
+return userdb
diff --git a/src/types_ext.cc b/src/types_ext.cc
index cd8c6ea..673fdfa 100644
--- a/src/types_ext.cc
+++ b/src/types_ext.cc
@@ -252,12 +252,10 @@ namespace UserDbReg{
   using A = DbAccessor;
 
   an<T> make(const string& db_name, const string& db_class) {
-    if (auto comp= UserDb::Require(db_class)){
-      return an<T>( comp->Create(db_name)) ;
-    }
-    else {
-      return {};
+    if (auto comp= Db::Require(db_class)) {
+      return an<T>(comp->Create(db_name));
     }
+    return {};
   }
 
   an<T> make_leveldb(const string& db_name) {
@@ -275,16 +273,6 @@ namespace UserDbReg{
     return {};
   }
 
-  bool Open(T &t) { return t.Open(); }
-  bool Close(T &t) { return t.Close(); }
-  bool OpenReadOnly(T &t) { return t.OpenReadOnly(); }
-  bool Erase(T &t, const string &key) { return t.Erase(key); }
-  bool Update(T &t, const string &key, const string &value) {
-    return t.Update(key, value);
-  }
-
-  an<A> Query(T &t, const string& key) { return t.Query(key); }
-
   static const luaL_Reg funcs[] = {
     {"UserDb", WRAP(make)},// an<Db> LevelDb( db_file, db_name)
     {"LevelDb", WRAP(make_leveldb)},// an<Db> LevelDb( db_file, db_name)
@@ -293,13 +281,13 @@ namespace UserDbReg{
   };
 
   static const luaL_Reg methods[] = {
-    {"open", WRAP(Open)},
-    {"open_read_only", WRAP(OpenReadOnly)},
-    {"close", WRAP(Close)},
-    {"query", WRAP(Query)}, // query(prefix_key) return DbAccessor
+    {"open", WRAPMEM(T, Open)},
+    {"open_read_only", WRAPMEM(T, OpenReadOnly)},
+    {"close", WRAPMEM(T, Close)},
+    {"query", WRAPMEM(T, Query)}, // query(prefix_key) return DbAccessor
     {"fetch", WRAP(fetch)},  //   fetch(key) return value
-    {"update", WRAP(Update)}, // update(key,value) return bool
-    {"erase", WRAP(Erase)}, // erase(key) return bool
+    {"update", WRAPMEM(T, Update)}, // update(key,value) return bool
+    {"erase", WRAPMEM(T, Erase)}, // erase(key) return bool
 
     {"loaded",WRAPMEM(T, loaded)},
     {"disable", WRAPMEM(T, disable)},