original_model_count支持len
避免在ModelCache为空时，repr时递归调用
Add repr(Foobar) # when 0 items
Fix str(item_id)
add model name prefix to _model_cache.cache_dir
ensure makedirs
Add _model_cache.dbpath
Fix InterfaceError: Error binding parameter 0 - probably unsupported type
Fix item_id1, item1 for iter(dict)