session.py

SQLAlchemy. 经典的Python ORM框架。学习必看。

        if type is None:
            try:
                self.commit()
            except:
                self.rollback()
                raise
        else:
            self.rollback()

class Session(object):
    """Encapsulates a set of objects being operated upon within an object-relational operation.

    The Session is the front end to SQLAlchemy's **Unit of Work** implementation. The concept
    behind Unit of Work is to track modifications to a field of objects, and then be able to
    flush those changes to the database in a single operation.

    SQLAlchemy's unit of work includes these functions:

    * The ability to track in-memory changes on scalar- and collection-based object
      attributes, such that database persistence operations can be assembled based on those
      changes.

    * The ability to organize individual SQL queries and population of newly generated
      primary and foreign key-holding attributes during a persist operation such that
      referential integrity is maintained at all times.

    * The ability to maintain insert ordering against the order in which new instances were
      added to the session.

    * an Identity Map, which is a dictionary keying instances to their unique primary key
      identity. This ensures that only one copy of a particular entity is ever present
      within the session, even if repeated load operations for the same entity occur. This
      allows many parts of an application to get a handle to a particular object without
      any chance of modifications going to two different places.

    When dealing with instances of mapped classes, an instance may be *attached* to a
    particular Session, else it is *unattached* . An instance also may or may not correspond
    to an actual row in the database. These conditions break up into four distinct states:

    * *Transient* - an instance that's not in a session, and is not saved to the database;
      i.e. it has no database identity. The only relationship such an object has to the ORM
      is that its class has a `mapper()` associated with it.

    * *Pending* - when you `save()` a transient instance, it becomes pending. It still
      wasn't actually flushed to the database yet, but it will be when the next flush
      occurs.

    * *Persistent* - An instance which is present in the session and has a record in the
      database. You get persistent instances by either flushing so that the pending
      instances become persistent, or by querying the database for existing instances (or
      moving persistent instances from other sessions into your local session).

    * *Detached* - an instance which has a record in the database, but is not in any
      session. Theres nothing wrong with this, and you can use objects normally when
      they're detached, **except** they will not be able to issue any SQL in order to load
      collections or attributes which are not yet loaded, or were marked as "expired".

    The session methods which control instance state include ``save()``, ``update()``,
    ``save_or_update()``, ``delete()``, ``merge()``, and ``expunge()``.

    The Session object is **not** threadsafe, particularly during flush operations.  A session
    which is only read from (i.e. is never flushed) can be used by concurrent threads if it's
    acceptable that some object instances may be loaded twice.

    The typical pattern to managing Sessions in a multi-threaded environment is either to use
    mutexes to limit concurrent access to one thread at a time, or more commonly to establish
    a unique session for every thread, using a threadlocal variable.  SQLAlchemy provides
    a thread-managed Session adapter, provided by the [sqlalchemy.orm#scoped_session()] function.
    """

    def __init__(self, bind=None, autoflush=True, transactional=False, twophase=False, echo_uow=False, weak_identity_map=True, binds=None, extension=None):
        """Construct a new Session.
        
        A session is usually constructed using the [sqlalchemy.orm#create_session()] function, 
        or its more "automated" variant [sqlalchemy.orm#sessionmaker()].

        autoflush
            When ``True``, all query operations will issue a ``flush()`` call to this
            ``Session`` before proceeding. This is a convenience feature so that
            ``flush()`` need not be called repeatedly in order for database queries to
            retrieve results. It's typical that ``autoflush`` is used in conjunction with
            ``transactional=True``, so that ``flush()`` is never called; you just call
            ``commit()`` when changes are complete to finalize all changes to the
            database.

        bind
            An optional ``Engine`` or ``Connection`` to which this ``Session`` should be
            bound. When specified, all SQL operations performed by this session will
            execute via this connectable.

        binds
            An optional dictionary, which contains more granular "bind" information than
            the ``bind`` parameter provides. This dictionary can map individual ``Table``
            instances as well as ``Mapper`` instances to individual ``Engine`` or
            ``Connection`` objects. Operations which proceed relative to a particular
            ``Mapper`` will consult this dictionary for the direct ``Mapper`` instance as
            well as the mapper's ``mapped_table`` attribute in order to locate an
            connectable to use. The full resolution is described in the ``get_bind()``
            method of ``Session``. Usage looks like::

                sess = Session(binds={
                    SomeMappedClass : create_engine('postgres://engine1'),
                    somemapper : create_engine('postgres://engine2'),
                    some_table : create_engine('postgres://engine3'),
                })

            Also see the ``bind_mapper()`` and ``bind_table()`` methods.

        echo_uow
            When ``True``, configure Python logging to dump all unit-of-work
            transactions. This is the equivalent of
            ``logging.getLogger('sqlalchemy.orm.unitofwork').setLevel(logging.DEBUG)``.

        extension
            An optional [sqlalchemy.orm.session#SessionExtension] instance, which will receive
            pre- and post- commit and flush events, as well as a post-rollback event.  User-
            defined code may be placed within these hooks using a user-defined subclass
            of ``SessionExtension``.

        transactional
            Set up this ``Session`` to automatically begin transactions. Setting this
            flag to ``True`` is the rough equivalent of calling ``begin()`` after each
            ``commit()`` operation, after each ``rollback()``, and after each
            ``close()``. Basically, this has the effect that all session operations are
            performed within the context of a transaction. Note that the ``begin()``
            operation does not immediately utilize any connection resources; only when
            connection resources are first required do they get allocated into a
            transactional context.

        twophase
            When ``True``, all transactions will be started using
            [sqlalchemy.engine_TwoPhaseTransaction]. During a ``commit()``, after
            ``flush()`` has been issued for all attached databases, the ``prepare()``
            method on each database's ``TwoPhaseTransaction`` will be called. This allows
            each database to roll back the entire transaction, before each transaction is
            committed.

        weak_identity_map
            When set to the default value of ``False``, a weak-referencing map is used;
            instances which are not externally referenced will be garbage collected
            immediately. For dereferenced instances which have pending changes present,
            the attribute management system will create a temporary strong-reference to
            the object which lasts until the changes are flushed to the database, at which
            point it's again dereferenced. Alternatively, when using the value ``True``,
            the identity map uses a regular Python dictionary to store instances. The
            session will maintain all instances present until they are removed using
            expunge(), clear(), or purge().
        """
        self.echo_uow = echo_uow
        self.weak_identity_map = weak_identity_map
        self.uow = unitofwork.UnitOfWork(self)
        self.identity_map = self.uow.identity_map

        self.bind = bind
        self.__binds = {}
        self.transaction = None
        self.hash_key = id(self)
        self.autoflush = autoflush
        self.transactional = transactional
        self.twophase = twophase
        self.extension = extension
        self._query_cls = query.Query
        self._mapper_flush_opts = {}

        if binds is not None:
            for mapperortable, value in binds.iteritems():
                if isinstance(mapperortable, type):
                    mapperortable = _class_mapper(mapperortable).base_mapper
                self.__binds[mapperortable] = value
                if isinstance(mapperortable, Mapper):
                    for t in mapperortable._all_tables:
                        self.__binds[t] = value

        if self.transactional:
            self.begin()
        _sessions[self.hash_key] = self

    def begin(self, **kwargs):
        """Begin a transaction on this Session."""

        if self.transaction is not None:
            self.transaction = self.transaction._begin(**kwargs)
        else:
            self.transaction = SessionTransaction(self, **kwargs)
        return self.transaction

    create_transaction = begin

    def begin_nested(self):
        """Begin a `nested` transaction on this Session.

        This utilizes a ``SAVEPOINT`` transaction for databases
        which support this feature.
        """

        return self.begin(nested=True)

    def rollback(self):
        """Rollback the current transaction in progress.

        If no transaction is in progress, this method is a
        pass-thru.
        """

        if self.transaction is None:
            pass
        else:
            self.transaction.rollback()
        # TODO: we can rollback attribute values.  however
        # we would want to expand attributes.py to be able to save *two* rollback points, one to the
        # last flush() and the other to when the object first entered the transaction.
        # [ticket:705]
        #attributes.rollback(*self.identity_map.values())
        if self.transaction is None and self.transactional:
            self.begin()

    def commit(self):
        """Commit the current transaction in progress.

        If no transaction is in progress, this method raises
        an InvalidRequestError.

        If the ``begin()`` method was called on this ``Session``
        additional times subsequent to its first call,
        ``commit()`` will not actually commit, and instead
        pops an internal SessionTransaction off its internal stack
        of transactions.  Only when the "root" SessionTransaction
        is reached does an actual database-level commit occur.
        """

        if self.transaction is None:
            if self.transactional:
                self.begin()
            else:
                raise exceptions.InvalidRequestError("No transaction is begun.")

        self.transaction.commit()
        if self.transaction is None and self.transactional:
            self.begin()
    
    def prepare(self):
        """Prepare the current transaction in progress for two phase commit.

        If no transaction is in progress, this method raises
        an InvalidRequestError.

        Only root transactions of two phase sessions can be prepared. If the current transaction is
        not such, an InvalidRequestError is raised.
        """
        if self.transaction is None:
            if self.transactional:
                self.begin()
            else:
                raise exceptions.InvalidRequestError("No transaction is begun.")

        self.transaction.prepare()

    def connection(self, mapper=None, **kwargs):
        """Return a ``Connection`` corresponding to this session's
        transactional context, if any.

        If this ``Session`` is transactional, the connection will be in
        the context of this session's transaction.  Otherwise, the
        connection is returned by the ``contextual_connect()`` method
        on the engine.

        The `mapper` argument is a class or mapper to which a bound engine
        will be located; use this when the Session itself is either bound
        to multiple engines or connections, or is not bound to any connectable.

        \**kwargs are additional arguments which will be passed to get_bind().
        See the get_bind() method for details.  Note that the ``ShardedSession``
        subclass takes a different get_bind() argument signature.
        """

        return self.__connection(self.get_bind(mapper, **kwargs))

    def __connection(self, engine, **kwargs):
        if self.transaction is not None:
            return self.transaction.get_or_add(engine)
        else:
            return engine.contextual_connect(**kwargs)

    def execute(self, clause, params=None, mapper=None, **kwargs):
        """Using the given mapper to identify the appropriate ``Engine``
        or ``Connection`` to be used for statement execution, execute the
        given ``ClauseElement`` using the provided parameter dictionary.

        Return a ``ResultProxy`` corresponding to the execution's results.

        If this method allocates a new ``Connection`` for the operation,
        then the ``ResultProxy`` 's ``close()`` method will release the
        resources of the underlying ``Connection``.
        """

        engine = self.get_bind(mapper, clause=clause, **kwargs)

        return self.__connection(engine, close_with_result=True).execute(clause, params or {})

    def scalar(self, clause, params=None, mapper=None, **kwargs):
        """Like execute() but return a scalar result."""

        engine = self.get_bind(mapper, clause=clause)

        return self.__connection(engine, close_with_result=True).scalar(clause, params or {}, **kwargs)

    def close(self):