python - SQLAlchemy __init__ 未运行

Question

我有以下代码:

session = scoped_session(sessionmaker(autocommit=False, autoflush=True, bind=engine))

Base = declarative_base()
Base.query = session.query_property()

class CommonBase(object):
  created_at = Column(DateTime, default=datetime.datetime.now)
  updated_at = Column(DateTime, default=datetime.datetime.now, onupdate=datetime.datetime.now)

class Look(Base, CommonBase):
  __tablename__ = "looks"
  id = Column(Integer, primary_key=True)

  def __init__(self):
    print "__init__ is run"
    Base.__init__(self)
    self.feedback = None

  def set_feedback(self, feedback):
    """Status can either be 1 for liked, 0 no response, or -1 disliked.
    """
    assert feedback in [1, 0, -1]
    self.feedback = feedback

  def get_feedback(self):
    return self.feedback

我收到以下错误:

Traceback (most recent call last):
  File "/Volumes/Data2/Dropbox/projects/Giordano/venv/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Volumes/Data2/Dropbox/projects/Giordano/venv/lib/python2.7/site-packages/flask/app.py", line 1689, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/Volumes/Data2/Dropbox/projects/Giordano/venv/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
    response = self.full_dispatch_request()
  File "/Volumes/Data2/Dropbox/projects/Giordano/venv/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Volumes/Data2/Dropbox/projects/Giordano/venv/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Volumes/Data2/Dropbox/projects/Giordano/venv/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Volumes/Data2/Dropbox/projects/Giordano/src/giordano/web/backend.py", line 94, in wrapped
    ret = f(*args, **kwargs)
  File "/Volumes/Data2/Dropbox/projects/Giordano/src/giordano/web/backend.py", line 81, in decorated
    return f(*args, **kwargs)
  File "/Volumes/Data2/Dropbox/projects/Giordano/src/giordano/web/backend.py", line 187, in next
    json_ret = ge.encode(results)     # automatically pulls the tags
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py", line 201, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py", line 264, in iterencode
    return _iterencode(o, 0)
  File "/Volumes/Data2/Dropbox/projects/Giordano/src/giordano/__init__.py", line 54, in default
    jsonable = self.convert_to_jsonable(obj)
  File "/Volumes/Data2/Dropbox/projects/Giordano/src/giordano/__init__.py", line 40, in convert_to_jsonable
    image_url=obj.image_url, feedback=obj.get_feedback())
  File "/Volumes/Data2/Dropbox/projects/Giordano/src/giordano/models.py", line 100, in get_feedback
    return self.feedback
AttributeError: 'Look' object has no attribute 'feedback'

在我看来，我的 __init__ 方法没有运行，因为我在日志中看不到任何打印语句。

有人可以解释为什么我的 __init__ 没有运行吗？我该怎么做？

Answer 1

查看 SQLAlchemy documentation on reconstruction :

The SQLAlchemy ORM does not call __init__ when recreating objects from database rows. The ORM’s process is somewhat akin to the Python standard library’s pickle module, invoking the low level __new__ method and then quietly restoring attributes directly on the instance rather than calling __init__.

If you need to do some setup on database-loaded instances before they’re ready to use, you can use the @reconstructor decorator to tag a method as the ORM counterpart to __init__. SQLAlchemy will call this method with no arguments every time it loads or reconstructs one of your instances. This is useful for recreating transient properties that are normally assigned in your __init__:

from sqlalchemy import orm

class MyMappedClass(object):
    def __init__(self, data):
        self.data = data
        # we need stuff on all instances, but not in the database.
        self.stuff = []

    @orm.reconstructor
    def init_on_load(self):
        self.stuff = []

When obj = MyMappedClass() is executed, Python calls the __init__ method as normal and the data argument is required. When instances are loaded during a Query operation as in query(MyMappedClass).one(), init_on_load is called.