在SQLAlchemy中查詢混合屬性

Question

我將文件路徑作爲相對路徑存儲在數據庫中，但是我在使用混合屬性映射時將其轉化爲絕對路徑。當我使用這個屬性查詢時，它會引發錯誤。這裏的模型：

class File(Base): 
    __tablename__ = 'files' 
    ... 

    _f_path = Column(Unicode(30)) 

    ... 

    @hybrid_property 
    def f_path(self): 
     env = shelve.open('environment') 
     return os.path.join(env['project_dir'], self._f_path) 

    @f_path.setter 
    def f_path(self, _f_path): 
     self._f_path = _f_path 

當我運行此查詢（其中REF是Unicode字符串）：

session.query(File).filter_by(f_path=ref).first() 

它給我這個錯誤：

File "/Users/Ben/Dropbox/Giraffe/giraffe_server/giraffe/file_handlers/maya.py", line 135, in process_file 
    rf = session.query(File).filter_by(f_path=str(ref)).first() 
    File "build/bdist.macosx-10.7-intel/egg/sqlalchemy/orm/query.py", line 1211, in filter_by 
    for key, value in kwargs.iteritems()] 
    File "build/bdist.macosx-10.7-intel/egg/sqlalchemy/orm/util.py", line 597, in _entity_descriptor 
    return getattr(entity, key) 
    File "build/bdist.macosx-10.7-intel/egg/sqlalchemy/ext/hybrid.py", line 681, in __get__ 
    return self.expr(owner) 
    File "/Users/Ben/Dropbox/Giraffe/giraffe_server/giraffe/model.py", line 133, in f_path 
    print "\n\n\n[model.py:File@f_path hybrid_property] returning: ", os.path.join(env['project_dir'], self._f_path) 
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 66, in join 
    if b.startswith('/'): 
    File "build/bdist.macosx-10.7-intel/egg/sqlalchemy/sql/expression.py", line 3426, in __nonzero__ 
    raise TypeError("Boolean value of this clause is not defined") 
TypeError: Boolean value of this clause is not defined 

Answer 1

您的混合屬性必須返回一個sql表達式;你的不會，它會返回一個python字符串。

要解決這種情況下，不這樣做的路徑加入蟒蛇，但在SQL表達式來代替：

return env['project_dir'] + os.path.sep + self._f_path 

將解析爲self._f_path.__radd__(result_of_project_dir_plus_os_path_sep)，這既可以在查詢和作爲使用返回值。