Querying a hybrid property in SQLAlchemy

问题:

I'm storing file paths as relative paths in the database, but I'm then using hybrid properties to turn in into an absolute path when its mapped. When I query using this property it throws an error. Here's the model:

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 

When I run this query (where ref is a unicode string):

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

It gives me this error:

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 

回答1:

Your hybrid property must return a sql expression; yours does not, it returns a python string instead.

To resolve that for this case, don't do the path join in python but in a SQL expression instead:

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

which will resolve to self._f_path.__radd__(result_of_project_dir_plus_os_path_sep), which can be used both in queries and as a return value.

文章来源: Querying a hybrid property in SQLAlchemy