# orm/session.py
# Copyright (C) 2005-2014 the SQLAlchemy authors and contributors <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php
"""Provides the Session class and related utilities."""



import weakref
from .. import util, sql, engine, exc as sa_exc
from ..sql import util as sql_util, expression
from . import (
    SessionExtension, attributes, exc, query,
    loading, identity
    )
from ..inspection import inspect
from .base import (
    object_mapper, class_mapper,
    _class_to_mapper, _state_mapper, object_state,
    _none_set, state_str, instance_str
    )
from .unitofwork import UOWTransaction
from . import state as statelib
import sys

__all__ = ['Session', 'SessionTransaction', 'SessionExtension', 'sessionmaker']

_sessions = weakref.WeakValueDictionary()
"""Weak-referencing dictionary of :class:`.Session` objects.
"""

def _state_session(state):
    """Given an :class:`.InstanceState`, return the :class:`.Session`
        associated, if any.
    """
    if state.session_id:
        try:
            return _sessions[state.session_id]
        except KeyError:
            pass
    return None



class _SessionClassMethods(object):
    """Class-level methods for :class:`.Session`, :class:`.sessionmaker`."""

    @classmethod
    def close_all(cls):
        """Close *all* sessions in memory."""

        for sess in _sessions.values():
            sess.close()

    @classmethod
    @util.dependencies("sqlalchemy.orm.util")
    def identity_key(cls, orm_util, *args, **kwargs):
        """Return an identity key.

        This is an alias of :func:`.util.identity_key`.

        """
        return orm_util.identity_key(*args, **kwargs)

    @classmethod
    def object_session(cls, instance):
        """Return the :class:`.Session` to which an object belongs.

        This is an alias of :func:`.object_session`.

        """

        return object_session(instance)


ACTIVE = util.symbol('ACTIVE')
PREPARED = util.symbol('PREPARED')
COMMITTED = util.symbol('COMMITTED')
DEACTIVE = util.symbol('DEACTIVE')
CLOSED = util.symbol('CLOSED')

class SessionTransaction(object):
    """A :class:`.Session`-level transaction.

    :class:`.SessionTransaction` is a mostly behind-the-scenes object
    not normally referenced directly by application code.   It coordinates
    among multiple :class:`.Connection` objects, maintaining a database
    transaction for each one individually, committing or rolling them
    back all at once.   It also provides optional two-phase commit behavior
    which can augment this coordination operation.

    The :attr:`.Session.transaction` attribute of :class:`.Session`
    refers to the current :class:`.SessionTransaction` object in use, if any.


    A :class:`.SessionTransaction` is associated with a :class:`.Session`
    in its default mode of ``autocommit=False`` immediately, associated
    with no database connections.  As the :class:`.Session` is called upon
    to emit SQL on behalf of various :class:`.Engine` or :class:`.Connection`
    objects, a corresponding :class:`.Connection` and associated
    :class:`.Transaction` is added to a collection within the
    :class:`.SessionTransaction` object, becoming one of the
    connection/transaction pairs maintained by the
    :class:`.SessionTransaction`.

    The lifespan of the :class:`.SessionTransaction` ends when the
    :meth:`.Session.commit`, :meth:`.Session.rollback` or
    :meth:`.Session.close` methods are called.  At this point, the
    :class:`.SessionTransaction` removes its association with its parent
    :class:`.Session`.   A :class:`.Session` that is in ``autocommit=False``
    mode will create a new :class:`.SessionTransaction` to replace it
    immediately, whereas a :class:`.Session` that's in ``autocommit=True``
    mode will remain without a :class:`.SessionTransaction` until the
    :meth:`.Session.begin` method is called.

    Another detail of :class:`.SessionTransaction` behavior is that it is
    capable of "nesting".  This means that the :meth:`.Session.begin` method
    can be called while an existing :class:`.SessionTransaction` is already
    present, producing a new :class:`.SessionTransaction` that temporarily
    replaces the parent :class:`.SessionTransaction`.   When a
    :class:`.SessionTransaction` is produced as nested, it assigns itself to
    the :attr:`.Session.transaction` attribute.  When it is ended via
    :meth:`.Session.commit` or :meth:`.Session.rollback`, it restores its
    parent :class:`.SessionTransaction` back onto the
    :attr:`.Session.transaction` attribute.  The behavior is effectively a
    stack, where :attr:`.Session.transaction` refers to the current head of
    the stack.

    The purpose of this stack is to allow nesting of
    :meth:`.Session.rollback` or :meth:`.Session.commit` calls in context
    with various flavors of :meth:`.Session.begin`. This nesting behavior
    applies to when :meth:`.Session.begin_nested` is used to emit a
    SAVEPOINT transaction, and is also used to produce a so-called
    "subtransaction" which allows a block of code to use a
    begin/rollback/commit sequence regardless of whether or not its enclosing
    code block has begun a transaction.  The :meth:`.flush` method, whether
    called explicitly or via autoflush, is the primary consumer of the
    "subtransaction" feature, in that it wishes to guarantee that it works
    within in a transaction block regardless of whether or not the
    :class:`.Session` is in transactional mode when the method is called.

    See also:

    :meth:`.Session.rollback`

    :meth:`.Session.commit`

    :meth:`.Session.begin`

    :meth:`.Session.begin_nested`

    :attr:`.Session.is_active`

    :meth:`.SessionEvents.after_commit`

    :meth:`.SessionEvents.after_rollback`

    :meth:`.SessionEvents.after_soft_rollback`

    """

    _rollback_exception = None

    def __init__(self, session, parent=None, nested=False):
        self.session = session
        self._connections = {}
        self._parent = parent
        self.nested = nested
        self._state = ACTIVE
        if not parent and nested:
            raise sa_exc.InvalidRequestError(
                "Can't start a SAVEPOINT transaction when no existing "
                "transaction is in progress")

        if self.session._enable_transaction_accounting:
            self._take_snapshot()

        if self.session.dispatch.after_transaction_create:
            self.session.dispatch.after_transaction_create(self.session, self)

    @property
    def is_active(self):
        return self.session is not None and self._state is ACTIVE

    def _assert_active(self, prepared_ok=False,
                        rollback_ok=False,
                        deactive_ok=False,
                        closed_msg="This transaction is closed"):
        if self._state is COMMITTED:
            raise sa_exc.InvalidRequestError(
                    "This session is in 'committed' state; no further "
                    "SQL can be emitted within this transaction."
                )
        elif self._state is PREPARED:
            if not prepared_ok:
                raise sa_exc.InvalidRequestError(
                        "This session is in 'prepared' state; no further "
                        "SQL can be emitted within this transaction."
                    )
        elif self._state is DEACTIVE:
            if not deactive_ok and not rollback_ok:
                if self._rollback_exception:
                    raise sa_exc.InvalidRequestError(
                        "This Session's transaction has been rolled back "
                        "due to a previous exception during flush."
                        " To begin a new transaction with this Session, "
                        "first issue Session.rollback()."
                        " Original exception was: %s"
                        % self._rollback_exception
                    )
                elif not deactive_ok:
                    raise sa_exc.InvalidRequestError(
                        "This Session's transaction has been rolled back "
                        "by a nested rollback() call.  To begin a new "
                        "transaction, issue Session.rollback() first."
                        )
        elif self._state is CLOSED:
            raise sa_exc.ResourceClosedError(closed_msg)

    @property
    def _is_transaction_boundary(self):
        return self.nested or not self._parent

    def connection(self, bindkey, **kwargs):
        self._assert_active()
        bind = self.session.get_bind(bindkey, **kwargs)
        return self._connection_for_bind(bind)

    def _begin(self, nested=False):
        self._assert_active()
        return SessionTransaction(
            self.session, self, nested=nested)

    def _iterate_parents(self, upto=None):
        if self._parent is upto:
            return (self,)
        else:
            if self._parent is None:
                raise sa_exc.InvalidRequestError(
                    "Transaction %s is not on the active transaction list" % (
                    upto))
            return (self,) + self._parent._iterate_parents(upto)

    def _take_snapshot(self):
        if not self._is_transaction_boundary:
            self._new = self._parent._new
            self._deleted = self._parent._deleted
            self._dirty = self._parent._dirty
            self._key_switches = self._parent._key_switches
            return

        if not self.session._flushing:
            self.session.flush()

        self._new = weakref.WeakKeyDictionary()
        self._deleted = weakref.WeakKeyDictionary()
        self._dirty = weakref.WeakKeyDictionary()
        self._key_switches = weakref.WeakKeyDictionary()

    def _restore_snapshot(self, dirty_only=False):
        assert self._is_transaction_boundary

        for s in set(self._new).union(self.session._new):
            self.session._expunge_state(s)
            if s.key:
                del s.key

        for s, (oldkey, newkey) in self._key_switches.items():
            self.session.identity_map.discard(s)
            s.key = oldkey
            self.session.identity_map.replace(s)

        for s in set(self._deleted).union(self.session._deleted):
            if s.deleted:
                #assert s in self._deleted
                del s.deleted
            self.session._update_impl(s, discard_existing=True)

        assert not self.session._deleted

        for s in self.session.identity_map.all_states():
            if not dirty_only or s.modified or s in self._dirty:
                s._expire(s.dict, self.session.identity_map._modified)

    def _remove_snapshot(self):
        assert self._is_transaction_boundary

        if not self.nested and self.session.expire_on_commit:
            for s in self.session.identity_map.all_states():
                s._expire(s.dict, self.session.identity_map._modified)
            for s in self._deleted:
                s.session_id = None
            self._deleted.clear()


    def _connection_for_bind(self, bind):
        self._assert_active()

        if bind in self._connections:
            return self._connections[bind][0]

        if self._parent:
            conn = self._parent._connection_for_bind(bind)
            if not self.nested:
                return conn
        else:
            if isinstance(bind, engine.Connection):
                conn = bind
                if conn.engine in self._connections:
                    raise sa_exc.InvalidRequestError(
                        "Session already has a Connection associated for the "
                        "given Connection's Engine")
            else:
                conn = bind.contextual_connect()

        if self.session.twophase and self._parent is None:
            transaction = conn.begin_twophase()
        elif self.nested:
            transaction = conn.begin_nested()
        else:
            transaction = conn.begin()

        self._connections[conn] = self._connections[conn.engine] = \
          (conn, transaction, conn is not bind)
        self.session.dispatch.after_begin(self.session, self, conn)
        return conn

    def prepare(self):
        if self._parent is not None or not self.session.twophase:
            raise sa_exc.InvalidRequestError(
                "'twophase' mode not enabled, or not root transaction; "
                "can't prepare.")
        self._prepare_impl()

    def _prepare_impl(self):
        self._assert_active()
        if self._parent is None or self.nested:
            self.session.dispatch.before_commit(self.session)

        stx = self.session.transaction
        if stx is not self:
            for subtransaction in stx._iterate_parents(upto=self):
                subtransaction.commit()

        if not self.session._flushing:
            for _flush_guard in range(100):
                if self.session._is_clean():
                    break
                self.session.flush()
            else:
                raise exc.FlushError(
                        "Over 100 subsequent flushes have occurred within "
                        "session.commit() - is an after_flush() hook "
                        "creating new objects?")

        if self._parent is None and self.session.twophase:
            try:
                for t in set(self._connections.values()):
                    t[1].prepare()
            except:
                with util.safe_reraise():
                    self.rollback()

        self._state = PREPARED

    def commit(self):
        self._assert_active(prepared_ok=True)
        if self._state is not PREPARED:
            self._prepare_impl()

        if self._parent is None or self.nested:
            for t in set(self._connections.values()):
                t[1].commit()

            self._state = COMMITTED
            self.session.dispatch.after_commit(self.session)

            if self.session._enable_transaction_accounting:
                self._remove_snapshot()

        self.close()
        return self._parent

    def rollback(self, _capture_exception=False):
        self._assert_active(prepared_ok=True, rollback_ok=True)

        stx = self.session.transaction
        if stx is not self:
            for subtransaction in stx._iterate_parents(upto=self):
                subtransaction.close()

        if self._state in (ACTIVE, PREPARED):
            for transaction in self._iterate_parents():
                if transaction._parent is None or transaction.nested:
                    transaction._rollback_impl()
                    transaction._state = DEACTIVE
                    break
                else:
                    transaction._state = DEACTIVE

        sess = self.session

        if self.session._enable_transaction_accounting and \
                not sess._is_clean():
            # if items were added, deleted, or mutated
            # here, we need to re-restore the snapshot
            util.warn(
                    "Session's state has been changed on "
                    "a non-active transaction - this state "
                    "will be discarded.")
            self._restore_snapshot(dirty_only=self.nested)

        self.close()
        if self._parent and _capture_exception:
            self._parent._rollback_exception = sys.exc_info()[1]

        sess.dispatch.after_soft_rollback(sess, self)

        return self._parent

    def _rollback_impl(self):
        for t in set(self._connections.values()):
            t[1].rollback()

        if self.session._enable_transaction_accounting:
            self._restore_snapshot(dirty_only=self.nested)

        self.session.dispatch.after_rollback(self.session)

    def close(self):
        self.session.transaction = self._parent
        if self._parent is None:
            for connection, transaction, autoclose in \
                    set(self._connections.values()):
                if autoclose:
                    connection.close()
                else:
                    transaction.close()

        self._state = CLOSED
        if self.session.dispatch.after_transaction_end:
            self.session.dispatch.after_transaction_end(self.session, self)

        if self._parent is None:
            if not self.session.autocommit:
                self.session.begin()
        self.session = None
        self._connections = None

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        self._assert_active(deactive_ok=True, prepared_ok=True)
        if self.session.transaction is None:
            return
        if type is None:
            try:
                self.commit()
            except:
                with util.safe_reraise():
                    self.rollback()
        else:
            self.rollback()


class Session(_SessionClassMethods):
    """Manages persistence operations for ORM-mapped objects.

    The Session's usage paradigm is described at :doc:`/orm/session`.


    """

    public_methods = (
        '__contains__', '__iter__', 'add', 'add_all', 'begin', 'begin_nested',
        'close', 'commit', 'connection', 'delete', 'execute', 'expire',
        'expire_all', 'expunge', 'expunge_all', 'flush', 'get_bind',
        'is_modified',
        'merge', 'query', 'refresh', 'rollback',
        'scalar')

    def __init__(self, bind=None, autoflush=True, expire_on_commit=True,
                _enable_transaction_accounting=True,
                 autocommit=False, twophase=False,
                 weak_identity_map=True, binds=None, extension=None,
                 info=None,
                 query_cls=query.Query):
        """Construct a new Session.

        See also the :class:`.sessionmaker` function which is used to
        generate a :class:`.Session`-producing callable with a given
        set of arguments.

        :param autocommit:

          .. warning::

             The autocommit flag is **not for general use**, and if it is used,
             queries should only be invoked within the span of a
             :meth:`.Session.begin` / :meth:`.Session.commit` pair.   Executing
             queries outside of a demarcated transaction is a legacy mode
             of usage, and can in some cases lead to concurrent connection
             checkouts.

          Defaults to ``False``. When ``True``, the
          :class:`.Session` does not keep a persistent transaction running, and
          will acquire connections from the engine on an as-needed basis,
          returning them immediately after their use. Flushes will begin and
          commit (or possibly rollback) their own transaction if no
          transaction is present. When using this mode, the
          :meth:`.Session.begin` method is used to explicitly start
          transactions.

          .. seealso::

            :ref:`session_autocommit`

        :param autoflush: When ``True``, all query operations will issue a
           :meth:`~.Session.flush` call to this ``Session`` before proceeding. 
           This is a convenience feature so that :meth:`~.Session.flush` need 
           not be called repeatedly in order for database queries to retrieve 
           results. It's typical that ``autoflush`` is used in conjunction with
           ``autocommit=False``. In this scenario, explicit calls to
           :meth:`~.Session.flush` are rarely needed; you usually only need to 
           call :meth:`~.Session.commit` (which flushes) to finalize changes.

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

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

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

          Also see the :meth:`.Session.bind_mapper`
          and :meth:`.Session.bind_table` methods.

        :param \class_: Specify an alternate class other than
           ``sqlalchemy.orm.session.Session`` which should be used by the
           returned class. This is the only argument that is local to the
           :class:`.sessionmaker` function, and is not sent directly to the
           constructor for ``Session``.

        :param _enable_transaction_accounting:  Defaults to ``True``.  A
           legacy-only flag which when ``False`` disables *all* 0.5-style
           object accounting on transaction boundaries, including auto-expiry
           of instances on rollback and commit, maintenance of the "new" and
           "deleted" lists upon rollback, and autoflush of pending changes upon
           :meth:`~.Session.begin`, all of which are interdependent.

        :param expire_on_commit:  Defaults to ``True``. When ``True``, all
           instances will be fully expired after each :meth:`~.commit`, 
           so that all attribute/object access subsequent to a completed 
           transaction will load from the most recent database state.

        :param extension: An optional
           :class:`~.SessionExtension` instance, or a list
           of such instances, which will receive pre- and post- commit and
           flush events, as well as a post-rollback event. **Deprecated.**
           Please see :class:`.SessionEvents`.

        :param info: optional dictionary of arbitrary data to be associated
           with this :class:`.Session`.  Is available via the :attr:`.Session.info`
           attribute.  Note the dictionary is copied at construction time so
           that modifications to the per-:class:`.Session` dictionary will be local
           to that :class:`.Session`.

           .. versionadded:: 0.9.0

        :param query_cls:  Class which should be used to create new Query
           objects, as returned by the :meth:`~.Session.query` method. Defaults 
           to :class:`.Query`.

        :param twophase:  When ``True``, all transactions will be started as
            a "two phase" transaction, i.e. using the "two phase" semantics
            of the database in use along with an XID.  During a 
            :meth:`~.commit`, after :meth:`~.flush` has been issued for all 
            attached databases, the :meth:`~.TwoPhaseTransaction.prepare` method
            on each database's :class:`.TwoPhaseTransaction` will be called. 
            This allows each database to roll back the entire transaction, 
            before each transaction is committed.

        :param weak_identity_map:  Defaults to ``True`` - when set to
           ``False``, objects placed in the :class:`.Session` will be
           strongly referenced until explicitly removed or the
           :class:`.Session` is closed.  **Deprecated** - this option
           is obsolete.

        """

        if weak_identity_map:
            self._identity_cls = identity.WeakInstanceDict
        else:
            util.warn_deprecated("weak_identity_map=False is deprecated.  "
                                    "This feature is not needed.")
            self._identity_cls = identity.StrongInstanceDict
        self.identity_map = self._identity_cls()

        self._new = {}   # InstanceState->object, strong refs object
        self._deleted = {}  # same
        self.bind = bind
        self.__binds = {}
        self._flushing = False
        self._warn_on_events = False
        self.transaction = None
        self.hash_key = _new_sessionid()
        self.autoflush = autoflush
        self.autocommit = autocommit
        self.expire_on_commit = expire_on_commit
        self._enable_transaction_accounting = _enable_transaction_accounting
        self.twophase = twophase
        self._query_cls = query_cls
        if info:
            self.info.update(info)

        if extension:
            for ext in util.to_list(extension):
                SessionExtension._adapt_listener(self, ext)

        if binds is not None:
            for mapperortable, bind in binds.items():
                insp = inspect(mapperortable)
                if insp.is_selectable:
                    self.bind_table(mapperortable, bind)
                elif insp.is_mapper:
                    self.bind_mapper(mapperortable, bind)
                else:
                    assert False


        if not self.autocommit:
            self.begin()
        _sessions[self.hash_key] = self

    connection_callable = None

    transaction = None
    """The current active or inactive :class:`.SessionTransaction`."""

    @util.memoized_property
    def info(self):
        """A user-modifiable dictionary.

        The initial value of this dictioanry can be populated using the
        ``info`` argument to the :class:`.Session` constructor or
        :class:`.sessionmaker` constructor or factory methods.  The dictionary
        here is always local to this :class:`.Session` and can be modified
        independently of all other :class:`.Session` objects.

        .. versionadded:: 0.9.0

        """
        return {}

    def begin(self, subtransactions=False, nested=False):
        """Begin a transaction on this :class:`.Session`.

        If this Session is already within a transaction, either a plain
        transaction or nested transaction, an error is raised, unless
        ``subtransactions=True`` or ``nested=True`` is specified.

        The ``subtransactions=True`` flag indicates that this
        :meth:`~.Session.begin` can create a subtransaction if a transaction
        is already in progress. For documentation on subtransactions, please
        see :ref:`session_subtransactions`.

        The ``nested`` flag begins a SAVEPOINT transaction and is equivalent
        to calling :meth:`~.Session.begin_nested`. For documentation on
        SAVEPOINT transactions, please see :ref:`session_begin_nested`.

        """
        if self.transaction is not None:
            if subtransactions or nested:
                self.transaction = self.transaction._begin(
                                        nested=nested)
            else:
                raise sa_exc.InvalidRequestError(
                    "A transaction is already begun.  Use "
                    "subtransactions=True to allow subtransactions.")
        else:
            self.transaction = SessionTransaction(
                self, nested=nested)
        return self.transaction  # needed for __enter__/__exit__ hook

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

        The target database(s) must support SQL SAVEPOINTs or a
        SQLAlchemy-supported vendor implementation of the idea.

        For documentation on SAVEPOINT
        transactions, please see :ref:`session_begin_nested`.

        """
        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-through.

        This method rolls back the current transaction or nested transaction
        regardless of subtransactions being in effect.  All subtransactions up
        to the first real transaction are closed.  Subtransactions occur when
        :meth:`.begin` is called multiple times.

        .. seealso::

            :ref:`session_rollback`

        """
        if self.transaction is None:
            pass
        else:
            self.transaction.rollback()

    def commit(self):
        """Flush pending changes and commit the current transaction.

        If no transaction is in progress, this method raises an
        :exc:`~sqlalchemy.exc.InvalidRequestError`.

        By default, the :class:`.Session` also expires all database
        loaded state on all ORM-managed attributes after transaction commit.
        This so that subsequent operations load the most recent
        data from the database.   This behavior can be disabled using
        the ``expire_on_commit=False`` option to :class:`.sessionmaker` or
        the :class:`.Session` constructor.

        If a subtransaction is in effect (which occurs when begin() is called
        multiple times), the subtransaction will be closed, and the next call
        to ``commit()`` will operate on the enclosing transaction.

        When using the :class:`.Session` in its default mode of
        ``autocommit=False``, a new transaction will
        be begun immediately after the commit, but note that the newly begun
        transaction does *not* use any connection resources until the first
        SQL is actually emitted.

        .. seealso::

            :ref:`session_committing`

        """
        if self.transaction is None:
            if not self.autocommit:
                self.begin()
            else:
                raise sa_exc.InvalidRequestError("No transaction is begun.")

        self.transaction.commit()

    def prepare(self):
        """Prepare the current transaction in progress for two phase commit.

        If no transaction is in progress, this method raises an
        :exc:`~sqlalchemy.exc.InvalidRequestError`.

        Only root transactions of two phase sessions can be prepared. If the
        current transaction is not such, an
        :exc:`~sqlalchemy.exc.InvalidRequestError` is raised.

        """
        if self.transaction is None:
            if not self.autocommit:
                self.begin()
            else:
                raise sa_exc.InvalidRequestError("No transaction is begun.")

        self.transaction.prepare()

    def connection(self, mapper=None, clause=None,
                        bind=None,
                        close_with_result=False,
                        **kw):
        """Return a :class:`.Connection` object corresponding to this
        :class:`.Session` object's transactional state.

        If this :class:`.Session` is configured with ``autocommit=False``,
        either the :class:`.Connection` corresponding to the current
        transaction is returned, or if no transaction is in progress, a new
        one is begun and the :class:`.Connection` returned (note that no
        transactional state is established with the DBAPI until the first
        SQL statement is emitted).

        Alternatively, if this :class:`.Session` is configured with
        ``autocommit=True``, an ad-hoc :class:`.Connection` is returned
        using :meth:`.Engine.contextual_connect` on the underlying
        :class:`.Engine`.

        Ambiguity in multi-bind or unbound :class:`.Session` objects can be
        resolved through any of the optional keyword arguments.   This
        ultimately makes usage of the :meth:`.get_bind` method for resolution.

        :param bind:
          Optional :class:`.Engine` to be used as the bind.  If
          this engine is already involved in an ongoing transaction,
          that connection will be used.  This argument takes precedence
          over ``mapper``, ``clause``.

        :param mapper:
          Optional :func:`.mapper` mapped class, used to identify
          the appropriate bind.  This argument takes precedence over
          ``clause``.

        :param clause:
            A :class:`.ClauseElement` (i.e. :func:`~.sql.expression.select`,
            :func:`~.sql.expression.text`,
            etc.) which will be used to locate a bind, if a bind
            cannot otherwise be identified.

        :param close_with_result: Passed to :meth:`.Engine.connect`, indicating
          the :class:`.Connection` should be considered "single use",
          automatically closing when the first result set is closed.  This
          flag only has an effect if this :class:`.Session` is configured with
          ``autocommit=True`` and does not already have a  transaction
          in progress.

        :param \**kw:
          Additional keyword arguments are sent to :meth:`get_bind()`,
          allowing additional arguments to be passed to custom
          implementations of :meth:`get_bind`.

        """
        if bind is None:
            bind = self.get_bind(mapper, clause=clause, **kw)

        return self._connection_for_bind(bind,
                                        close_with_result=close_with_result)

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

    def execute(self, clause, params=None, mapper=None, bind=None, **kw):
        """Execute a SQL expression construct or string statement within
        the current transaction.

        Returns a :class:`.ResultProxy` representing
        results of the statement execution, in the same manner as that of an
        :class:`.Engine` or
        :class:`.Connection`.

        E.g.::

            result = session.execute(
                        user_table.select().where(user_table.c.id == 5)
                    )

        :meth:`~.Session.execute` accepts any executable clause construct, such
        as :func:`~.sql.expression.select`,
        :func:`~.sql.expression.insert`,
        :func:`~.sql.expression.update`,
        :func:`~.sql.expression.delete`, and
        :func:`~.sql.expression.text`.  Plain SQL strings can be passed
        as well, which in the case of :meth:`.Session.execute` only
        will be interpreted the same as if it were passed via a
        :func:`~.expression.text` construct.  That is, the following usage::

            result = session.execute(
                        "SELECT * FROM user WHERE id=:param",
                        {"param":5}
                    )

        is equivalent to::

            from sqlalchemy import text
            result = session.execute(
                        text("SELECT * FROM user WHERE id=:param"),
                        {"param":5}
                    )

        The second positional argument to :meth:`.Session.execute` is an
        optional parameter set.  Similar to that of
        :meth:`.Connection.execute`, whether this is passed as a single
        dictionary, or a list of dictionaries, determines whether the DBAPI
        cursor's ``execute()`` or ``executemany()`` is used to execute the
        statement.   An INSERT construct may be invoked for a single row::

            result = session.execute(users.insert(), {"id": 7, "name": "somename"})

        or for multiple rows::

            result = session.execute(users.insert(), [
                                    {"id": 7, "name": "somename7"},
                                    {"id": 8, "name": "somename8"},
                                    {"id": 9, "name": "somename9"}
                                ])

        The statement is executed within the current transactional context of
        this :class:`.Session`.   The :class:`.Connection` which is used
        to execute the statement can also be acquired directly by
        calling the :meth:`.Session.connection` method.  Both methods use
        a rule-based resolution scheme in order to determine the
        :class:`.Connection`, which in the average case is derived directly
        from the "bind" of the :class:`.Session` itself, and in other cases
        can be based on the :func:`.mapper`
        and :class:`.Table` objects passed to the method; see the documentation
        for :meth:`.Session.get_bind` for a full description of this scheme.

        The :meth:`.Session.execute` method does *not* invoke autoflush.

        The :class:`.ResultProxy` returned by the :meth:`.Session.execute`
        method is returned with the "close_with_result" flag set to true;
        the significance of this flag is that if this :class:`.Session` is
        autocommitting and does not have a transaction-dedicated
        :class:`.Connection` available, a temporary :class:`.Connection` is
        established for the statement execution, which is closed (meaning,
        returned to the connection pool) when the :class:`.ResultProxy` has
        consumed all available data. This applies *only* when the
        :class:`.Session` is configured with autocommit=True and no
        transaction has been started.

        :param clause:
            An executable statement (i.e. an :class:`.Executable` expression
            such as :func:`.expression.select`) or string SQL statement
            to be executed.

        :param params:
            Optional dictionary, or list of dictionaries, containing
            bound parameter values.   If a single dictionary, single-row
            execution occurs; if a list of dictionaries, an
            "executemany" will be invoked.  The keys in each dictionary
            must correspond to parameter names present in the statement.

        :param mapper:
          Optional :func:`.mapper` or mapped class, used to identify
          the appropriate bind.  This argument takes precedence over
          ``clause`` when locating a bind.   See :meth:`.Session.get_bind`
          for more details.

        :param bind:
          Optional :class:`.Engine` to be used as the bind.  If
          this engine is already involved in an ongoing transaction,
          that connection will be used.  This argument takes
          precedence over ``mapper`` and ``clause`` when locating
          a bind.

        :param \**kw:
          Additional keyword arguments are sent to :meth:`.Session.get_bind()`
          to allow extensibility of "bind" schemes.

        .. seealso::

            :ref:`sqlexpression_toplevel` - Tutorial on using Core SQL
            constructs.

            :ref:`connections_toplevel` - Further information on direct
            statement execution.

            :meth:`.Connection.execute` - core level statement execution
            method, which is :meth:`.Session.execute` ultimately uses
            in order to execute the statement.

        """
        clause = expression._literal_as_text(clause)

        if bind is None:
            bind = self.get_bind(mapper, clause=clause, **kw)

        return self._connection_for_bind(bind, close_with_result=True).execute(
            clause, params or {})

    def scalar(self, clause, params=None, mapper=None, bind=None, **kw):
        """Like :meth:`~.Session.execute` but return a scalar result."""

        return self.execute(
            clause, params=params, mapper=mapper, bind=bind, **kw).scalar()

    def close(self):
        """Close this Session.

        This clears all items and ends any transaction in progress.

        If this session were created with ``autocommit=False``, a new
        transaction is immediately begun.  Note that this new transaction does
        not use any connection resources until they are first needed.

        """
        self.expunge_all()
        if self.transaction is not None:
            for transaction in self.transaction._iterate_parents():
                transaction.close()

    def expunge_all(self):
        """Remove all object instances from this ``Session``.

        This is equivalent to calling ``expunge(obj)`` on all objects in this
        ``Session``.

        """
        for state in self.identity_map.all_states() + list(self._new):
            state._detach()

        self.identity_map = self._identity_cls()
        self._new = {}
        self._deleted = {}

    # TODO: need much more test coverage for bind_mapper() and similar !
    # TODO: + crystallize + document resolution order
    #       vis. bind_mapper/bind_table

    def bind_mapper(self, mapper, bind):
        """Bind operations for a mapper to a Connectable.

        mapper
          A mapper instance or mapped class

        bind
          Any Connectable: a :class:`.Engine` or :class:`.Connection`.

        All subsequent operations involving this mapper will use the given
        `bind`.

        """
        if isinstance(mapper, type):
            mapper = class_mapper(mapper)

        self.__binds[mapper.base_mapper] = bind
        for t in mapper._all_tables:
            self.__binds[t] = bind

    def bind_table(self, table, bind):
        """Bind operations on a Table to a Connectable.

        table
          A :class:`.Table` instance

        bind
          Any Connectable: a :class:`.Engine` or :class:`.Connection`.

        All subsequent operations involving this :class:`.Table` will use the 
        given `bind`.

        """
        self.__binds[table] = bind

    def get_bind(self, mapper=None, clause=None):
        """Return a "bind" to which this :class:`.Session` is bound.

        The "bind" is usually an instance of :class:`.Engine`,
        except in the case where the :class:`.Session` has been
        explicitly bound directly to a :class:`.Connection`.

        For a multiply-bound or unbound :class:`.Session`, the
        ``mapper`` or ``clause`` arguments are used to determine the
        appropriate bind to return.

        Note that the "mapper" argument is usually present
        when :meth:`.Session.get_bind` is called via an ORM
        operation such as a :meth:`.Session.query`, each
        individual INSERT/UPDATE/DELETE operation within a
        :meth:`.Session.flush`, call, etc.

        The order of resolution is:

        1. if mapper given and session.binds is present,
           locate a bind based on mapper.
        2. if clause given and session.binds is present,
           locate a bind based on :class:`.Table` objects
           found in the given clause present in session.binds.
        3. if session.bind is present, return that.
        4. if clause given, attempt to return a bind
           linked to the :class:`.MetaData` ultimately
           associated with the clause.
        5. if mapper given, attempt to return a bind
           linked to the :class:`.MetaData` ultimately
           associated with the :class:`.Table` or other
           selectable to which the mapper is mapped.
        6. No bind can be found, :exc:`~sqlalchemy.exc.UnboundExecutionError`
           is raised.

        :param mapper:
          Optional :func:`.mapper` mapped class or instance of
          :class:`.Mapper`.   The bind can be derived from a :class:`.Mapper`
          first by consulting the "binds" map associated with this
          :class:`.Session`, and secondly by consulting the :class:`.MetaData`
          associated with the :class:`.Table` to which the :class:`.Mapper`
          is mapped for a bind.

        :param clause:
            A :class:`.ClauseElement` (i.e. :func:`~.sql.expression.select`,
            :func:`~.sql.expression.text`,
            etc.).  If the ``mapper`` argument is not present or could not
            produce a bind, the given expression construct will be searched
            for a bound element, typically a :class:`.Table` associated with
            bound :class:`.MetaData`.

        """
        if mapper is clause is None:
            if self.bind:
                return self.bind
            else:
                raise sa_exc.UnboundExecutionError(
                    "This session is not bound to a single Engine or "
                    "Connection, and no context was provided to locate "
                    "a binding.")

        c_mapper = mapper is not None and _class_to_mapper(mapper) or None

        # manually bound?
        if self.__binds:
            if c_mapper:
                if c_mapper.base_mapper in self.__binds:
                    return self.__binds[c_mapper.base_mapper]
                elif c_mapper.mapped_table in self.__binds:
                    return self.__binds[c_mapper.mapped_table]
            if clause is not None:
                for t in sql_util.find_tables(clause, include_crud=True):
                    if t in self.__binds:
                        return self.__binds[t]

        if self.bind:
            return self.bind

        if isinstance(clause, sql.expression.ClauseElement) and clause.bind:
            return clause.bind

        if c_mapper and c_mapper.mapped_table.bind:
            return c_mapper.mapped_table.bind

        context = []
        if mapper is not None:
            context.append('mapper %s' % c_mapper)
        if clause is not None:
            context.append('SQL expression')

        raise sa_exc.UnboundExecutionError(
            "Could not locate a bind configured on %s or this Session" % (
            ', '.join(context)))

    def query(self, *entities, **kwargs):
        """Return a new :class:`.Query` object corresponding to this 
        :class:`.Session`."""

        return self._query_cls(entities, self, **kwargs)

    @property
    @util.contextmanager
    def no_autoflush(self):
        """Return a context manager that disables autoflush.

        e.g.::

            with session.no_autoflush:

                some_object = SomeClass()
                session.add(some_object)
                # won't autoflush
                some_object.related_thing = session.query(SomeRelated).first()

        Operations that proceed within the ``with:`` block
        will not be subject to flushes occurring upon query
        access.  This is useful when initializing a series
        of objects which involve existing database queries,
        where the uncompleted object should not yet be flushed.

        .. versionadded:: 0.7.6

        """
        autoflush = self.autoflush
        self.autoflush = False
        yield self
        self.autoflush = autoflush

    def _autoflush(self):
        if self.autoflush and not self._flushing:
            try:
                self.flush()
            except sa_exc.StatementError as e:
                # note we are reraising StatementError as opposed to
                # raising FlushError with "chaining" to remain compatible
                # with code that catches StatementError, IntegrityError,
                # etc.
                e.add_detail(
                        "raised as a result of Query-invoked autoflush; "
                        "consider using a session.no_autoflush block if this "
                        "flush is occurring prematurely")
                util.raise_from_cause(e)

    def refresh(self, instance, attribute_names=None, lockmode=None):
        """Expire and refresh the attributes on the given instance.

        A query will be issued to the database and all attributes will be
        refreshed with their current database value.

        Lazy-loaded relational attributes will remain lazily loaded, so that
        the instance-wide refresh operation will be followed immediately by
        the lazy load of that attribute.

        Eagerly-loaded relational attributes will eagerly load within the
        single refresh operation.

        Note that a highly isolated transaction will return the same values as
        were previously read in that same transaction, regardless of changes
        in database state outside of that transaction - usage of
        :meth:`~Session.refresh` usually only makes sense if non-ORM SQL
        statement were emitted in the ongoing transaction, or if autocommit
        mode is turned on.

        :param attribute_names: optional.  An iterable collection of
          string attribute names indicating a subset of attributes to
          be refreshed.

        :param lockmode: Passed to the :class:`~sqlalchemy.orm.query.Query`
          as used by :meth:`~sqlalchemy.orm.query.Query.with_lockmode`.

        .. seealso::

            :ref:`session_expire` - introductory material

            :meth:`.Session.expire`

            :meth:`.Session.expire_all`

        """
        try:
            state = attributes.instance_state(instance)
        except exc.NO_STATE:
            raise exc.UnmappedInstanceError(instance)

        self._expire_state(state, attribute_names)

        if loading.load_on_ident(
                self.query(object_mapper(instance)),
                state.key, refresh_state=state,
                lockmode=lockmode,
                only_load_props=attribute_names) is None:
            raise sa_exc.InvalidRequestError(
                "Could not refresh instance '%s'" %
                instance_str(instance))

    def expire_all(self):
        """Expires all persistent instances within this Session.

        When any attributes on a persistent instance is next accessed,
        a query will be issued using the
        :class:`.Session` object's current transactional context in order to
        load all expired attributes for the given instance.   Note that
        a highly isolated transaction will return the same values as were
        previously read in that same transaction, regardless of changes
        in database state outside of that transaction.

        To expire individual objects and individual attributes
        on those objects, use :meth:`Session.expire`.

        The :class:`.Session` object's default behavior is to
        expire all state whenever the :meth:`Session.rollback`
        or :meth:`Session.commit` methods are called, so that new
        st…