flask_sqlalchemy.py

# -*- coding: utf-8 -*-
"""
    flaskext.sqlalchemy
    ~~~~~~~~~~~~~~~~~~~

    Adds basic SQLAlchemy support to your application.

    :copyright: (c) 2012 by Armin Ronacher.
    :license: BSD, see LICENSE for more details.
"""
from __future__ import with_statement, absolute_import
from distutils.version import StrictVersion
import os
import re
import sys
import time
import functools
import sqlalchemy
from math import ceil
from functools import partial
from flask import _request_ctx_stack, abort
from flask.signals import Namespace
from operator import itemgetter
from threading import Lock
from sqlalchemy import orm
from sqlalchemy.orm.exc import UnmappedClassError
from sqlalchemy.orm.session import Session
from sqlalchemy.orm.interfaces import MapperExtension, SessionExtension, \
     EXT_CONTINUE
from sqlalchemy.interfaces import ConnectionProxy
from sqlalchemy.engine.url import make_url
from sqlalchemy.ext.declarative import declarative_base, DeclarativeMeta
from sqlalchemy.util import to_list

# the best timer function for the platform
if sys.platform == 'win32':
    _timer = time.clock
else:
    _timer = time.time

try:
    from flask import _app_ctx_stack
except ImportError:
    _app_ctx_stack = None


# Which stack should we use?  _app_ctx_stack is new in 0.9
connection_stack = _app_ctx_stack or _request_ctx_stack


_camelcase_re = re.compile(r'([A-Z]+)(?=[a-z0-9])')
_signals = Namespace()


models_committed = _signals.signal('models-committed')
before_models_committed = _signals.signal('before-models-committed')


def _make_table(db):
    def _make_table(*args, **kwargs):
        if len(args) > 1 and isinstance(args[1], db.Column):
            args = (args[0], db.metadata) + args[1:]
        info = kwargs.pop('info', None) or {}
        info.setdefault('bind_key', None)
        kwargs['info'] = info
        return sqlalchemy.Table(*args, **kwargs)
    return _make_table


def _set_default_query_class(d):
    if 'query_class' not in d:
        d['query_class'] = BaseQuery


def _wrap_with_default_query_class(fn):
    @functools.wraps(fn)
    def newfn(*args, **kwargs):
        _set_default_query_class(kwargs)
        if "backref" in kwargs:
            backref = kwargs['backref']
            if isinstance(backref, basestring):
                backref = (backref, {})
            _set_default_query_class(backref[1])
        return fn(*args, **kwargs)
    return newfn


def _include_sqlalchemy(obj):
    for module in sqlalchemy, sqlalchemy.orm:
        for key in module.__all__:
            if not hasattr(obj, key):
                setattr(obj, key, getattr(module, key))
    # Note: obj.Table does not attempt to be a SQLAlchemy Table class.
    obj.Table = _make_table(obj)
    obj.mapper = signalling_mapper
    obj.relationship = _wrap_with_default_query_class(obj.relationship)
    obj.relation = _wrap_with_default_query_class(obj.relation)
    obj.dynamic_loader = _wrap_with_default_query_class(obj.dynamic_loader)


class _DebugQueryTuple(tuple):
    statement = property(itemgetter(0))
    parameters = property(itemgetter(1))
    start_time = property(itemgetter(2))
    end_time = property(itemgetter(3))
    context = property(itemgetter(4))
    bind = property(itemgetter(5))

    @property
    def duration(self):
        return self.end_time - self.start_time

    def __repr__(self):
        return '<query statement="%s" parameters=%r duration=%.03f bind=%r>' % (
            self.statement,
            self.parameters,
            self.duration,
            self.bind
        )


def _calling_context(app_path):
    frm = sys._getframe(1)
    while frm.f_back is not None:
        name = frm.f_globals.get('__name__')
        if name and (name == app_path or name.startswith(app_path + '.')):
            funcname = frm.f_code.co_name
            return '%s:%s (%s)' % (
                frm.f_code.co_filename,
                frm.f_lineno,
                funcname
            )
        frm = frm.f_back
    return '<unknown>'


def _add_debug_query(statement, parameters, start, app_package, bind):
    """Remember query debug information"""
    ctx = connection_stack.top
    if ctx is not None:
        queries = getattr(ctx, 'sqlalchemy_queries', None)
        if queries is None:
            queries = []
            setattr(ctx, 'sqlalchemy_queries', queries)
        queries.append(_DebugQueryTuple((
            statement, parameters, start, _timer(),
            _calling_context(app_package), bind)))


class _ConnectionDebugProxy(ConnectionProxy):
    """Helps debugging the database."""

    def __init__(self, import_name, bind):
        self.app_package = import_name
        self.bind = bind

    def cursor_execute(self, execute, cursor, statement, parameters,
                       context, executemany):
        start = _timer()
        try:
            return execute(cursor, statement, parameters, context)
        finally:
            _add_debug_query(statement, parameters, start,
                self.app_package, self.bind)


class _QueryDebugger(ConnectionProxy):
    """Helps debugging the database."""

    _instance = None

    def __new__(cls, *args, **kwargs):
        """Use singleton to make sure that handlers are attached only once"""
        if not cls._instance:
            cls._instance = super(_QueryDebugger, cls).__new__(
                cls, *args, **kwargs)

            from sqlalchemy import event
            from sqlalchemy.engine import Engine

            @event.listens_for(Engine, "before_cursor_execute")
            def before_cursor_execute(conn, cursor, statement,
                                      parameters, context, executemany):
                context._query_start_time = _timer()

            @event.listens_for(Engine, "after_cursor_execute")
            def after_cursor_execute(conn, cursor, statement,
                                     parameters, context, executemany):
                app_packege = cls._instance.engines[conn.engine]['app_package']
                bind = cls._instance.engines[conn.engine]['bind']
                _add_debug_query(statement, parameters,
                    context._query_start_time, app_packege, bind)

        return cls._instance

    engines = {}

    def __init__(self, engine, import_name, bind):
        """Remember import name and bind for an engine"""
        self.engines[engine] = {'app_package': import_name, 'bind': bind}


class _SignalTrackingMapperExtension(MapperExtension):

    def after_delete(self, mapper, connection, instance):
        return self._record(mapper, instance, 'delete')

    def after_insert(self, mapper, connection, instance):
        return self._record(mapper, instance, 'insert')

    def after_update(self, mapper, connection, instance):
        return self._record(mapper, instance, 'update')

    def _record(self, mapper, model, operation):
        pk = tuple(mapper.primary_key_from_instance(model))
        orm.object_session(model)._model_changes[pk] = (model, operation)
        return EXT_CONTINUE


class _SignallingSessionExtension(SessionExtension):

    def before_commit(self, session):
        d = session._model_changes
        if d:
            before_models_committed.send(session.app, changes=d.values())
        return EXT_CONTINUE

    def after_commit(self, session):
        d = session._model_changes
        if d:
            models_committed.send(session.app, changes=d.values())
            d.clear()
        return EXT_CONTINUE

    def after_rollback(self, session):
        session._model_changes.clear()
        return EXT_CONTINUE


class _SignallingSession(Session):

    def __init__(self, db, autocommit=False, autoflush=False, **options):
        self.app = db.get_app()
        self._model_changes = {}
        Session.__init__(self, autocommit=autocommit, autoflush=autoflush,
                         extension=db.session_extensions,
                         bind=db.engine,
                         binds=db.get_binds(self.app), **options)

    def get_bind(self, mapper, clause=None):
        # mapper is None if someone tries to just get a connection
        if mapper is not None:
            info = getattr(mapper.mapped_table, 'info', {})
            bind_key = info.get('bind_key')
            if bind_key is not None:
                state = get_state(self.app)
                return state.db.get_engine(self.app, bind=bind_key)
        return Session.get_bind(self, mapper, clause)


def get_debug_queries():
    """In debug mode Flask-SQLAlchemy will log all the SQL queries sent
    to the database.  This information is available until the end of request
    which makes it possible to easily ensure that the SQL generated is the
    one expected on errors or in unittesting.  If you don't want to enable
    the DEBUG mode for your unittests you can also enable the query
    recording by setting the ``'SQLALCHEMY_RECORD_QUERIES'`` config variable
    to `True`.  This is automatically enabled if Flask is in testing mode.

    The value returned will be a list of named tuples with the following
    attributes:

    `statement`
        The SQL statement issued

    `parameters`
        The parameters for the SQL statement

    `start_time` / `end_time`
        Time the query started / the results arrived.  Please keep in mind
        that the timer function used depends on your platform. These
        values are only useful for sorting or comparing.  They do not
        necessarily represent an absolute timestamp.

    `duration`
        Time the query took in seconds

    `context`
        A string giving a rough estimation of where in your application
        query was issued.  The exact format is undefined so don't try
        to reconstruct filename or function name.
    """
    return getattr(connection_stack.top, 'sqlalchemy_queries', [])


class Pagination(object):
    """Internal helper class returned by :meth:`BaseQuery.paginate`.  You
    can also construct it from any other SQLAlchemy query object if you are
    working with other libraries.  Additionally it is possible to pass `None`
    as query object in which case the :meth:`prev` and :meth:`next` will
    no longer work.
    """

    def __init__(self, query, page, per_page, total, items):
        #: the unlimited query object that was used to create this
        #: pagination object.
        self.query = query
        #: the current page number (1 indexed)
        self.page = page
        #: the number of items to be displayed on a page.
        self.per_page = per_page
        #: the total number of items matching the query
        self.total = total
        #: the items for the current page
        self.items = items

    @property
    def pages(self):
        """The total number of pages"""
        return int(ceil(self.total / float(self.per_page)))

    def prev(self, error_out=False):
        """Returns a :class:`Pagination` object for the previous page."""
        assert self.query is not None, 'a query object is required ' \
                                       'for this method to work'
        return self.query.paginate(self.page - 1, self.per_page, error_out)

    @property
    def prev_num(self):
        """Number of the previous page."""
        return self.page - 1

    @property
    def has_prev(self):
        """True if a previous page exists"""
        return self.page > 1

    def next(self, error_out=False):
        """Returns a :class:`Pagination` object for the next page."""
        assert self.query is not None, 'a query object is required ' \
                                       'for this method to work'
        return self.query.paginate(self.page + 1, self.per_page, error_out)

    @property
    def has_next(self):
        """True if a next page exists."""
        return self.page < self.pages

    @property
    def next_num(self):
        """Number of the next page"""
        return self.page + 1

    def iter_pages(self, left_edge=2, left_current=2,
                   right_current=5, right_edge=2):
        """Iterates over the page numbers in the pagination.  The four
        parameters control the thresholds how many numbers should be produced
        from the sides.  Skipped page numbers are represented as `None`.
        This is how you could render such a pagination in the templates:

        .. sourcecode:: html+jinja

            {% macro render_pagination(pagination, endpoint) %}
              <div class=pagination>
              {%- for page in pagination.iter_pages() %}
                {% if page %}
                  {% if page != pagination.page %}
                    <a href="{{ url_for(endpoint, page=page) }}">{{ page }}</a>
                  {% else %}
                    <strong>{{ page }}</strong>
                  {% endif %}
                {% else %}
                  <span class=ellipsis>…</span>
                {% endif %}
              {%- endfor %}
              </div>
            {% endmacro %}
        """
        last = 0
        for num in xrange(1, self.pages + 1):
            if num <= left_edge or \
               (num > self.page - left_current - 1 and \
                num < self.page + right_current) or \
               num > self.pages - right_edge:
                if last + 1 != num:
                    yield None
                yield num
                last = num


class BaseQuery(orm.Query):
    """The default query object used for models, and exposed as
    :attr:`~SQLAlchemy.Query`. This can be subclassed and
    replaced for individual models by setting the :attr:`~Model.query_class`
    attribute.  This is a subclass of a standard SQLAlchemy
    :class:`~sqlalchemy.orm.query.Query` class and has all the methods of a
    standard query as well.
    """

    def get_or_404(self, ident):
        """Like :meth:`get` but aborts with 404 if not found instead of
        returning `None`.
        """
        rv = self.get(ident)
        if rv is None:
            abort(404)
        return rv

    def first_or_404(self):
        """Like :meth:`first` but aborts with 404 if not found instead of
        returning `None`.
        """
        rv = self.first()
        if rv is None:
            abort(404)
        return rv

    def paginate(self, page, per_page=20, error_out=True):
        """Returns `per_page` items from page `page`.  By default it will
        abort with 404 if no items were found and the page was larger than
        1.  This behavor can be disabled by setting `error_out` to `False`.

        Returns an :class:`Pagination` object.
        """
        if error_out and page < 1:
            abort(404)
        items = self.limit(per_page).offset((page - 1) * per_page).all()
        if not items and page != 1 and error_out:
            abort(404)
        return Pagination(self, page, per_page, self.count(), items)


class _QueryProperty(object):

    def __init__(self, sa):
        self.sa = sa

    def __get__(self, obj, type):
        try:
            mapper = orm.class_mapper(type)
            if mapper:
                return type.query_class(mapper, session=self.sa.session())
        except UnmappedClassError:
            return None


def _record_queries(app):
    if app.debug:
        return True
    rq = app.config['SQLALCHEMY_RECORD_QUERIES']
    if rq is not None:
        return rq
    return bool(app.config.get('TESTING'))


class _EngineConnector(object):

    def __init__(self, sa, app, bind=None):
        self._sa = sa
        self._app = app
        self._engine = None
        self._connected_for = None
        self._bind = bind
        self._lock = Lock()

    def get_uri(self):
        if self._bind is None:
            return self._app.config['SQLALCHEMY_DATABASE_URI']
        binds = self._app.config.get('SQLALCHEMY_BINDS') or ()
        assert self._bind in binds, \
            'Bind %r is not specified.  Set it in the SQLALCHEMY_BINDS ' \
            'configuration variable' % self._bind
        return binds[self._bind]

    def get_engine(self):
        with self._lock:
            uri = self.get_uri()
            echo = self._app.config['SQLALCHEMY_ECHO']
            if (uri, echo) == self._connected_for:
                return self._engine
            info = make_url(uri)
            options = {'convert_unicode': True}
            self._sa.apply_pool_defaults(self._app, options)
            self._sa.apply_driver_hacks(self._app, info, options)
            if (_record_queries(self._app) and
                    StrictVersion(sqlalchemy.__version__) <
                        StrictVersion('0.7.0')):
                options['proxy'] = _ConnectionDebugProxy(self._app.import_name,
                    self._bind)
            if echo:
                options['echo'] = True
            self._engine = rv = sqlalchemy.create_engine(info, **options)
            self._connected_for = (uri, echo)
            if (_record_queries(self._app) and
                    StrictVersion(sqlalchemy.__version__) >=
                        StrictVersion('0.7.0')):
                _QueryDebugger(rv, self._app.import_name, self._bind)
            return rv


def _defines_primary_key(d):
    """Figures out if the given dictonary defines a primary key column."""
    return any(v.primary_key for k, v in d.iteritems()
               if isinstance(v, sqlalchemy.Column))


class _BoundDeclarativeMeta(DeclarativeMeta):

    def __new__(cls, name, bases, d):
        tablename = d.get('__tablename__')

        # generate a table name automatically if it's missing and the
        # class dictionary declares a primary key.  We cannot always
        # attach a primary key to support model inheritance that does
        # not use joins.  We also don't want a table name if a whole
        # table is defined
        if not tablename and d.get('__table__') is None and \
           _defines_primary_key(d):
            def _join(match):
                word = match.group()
                if len(word) > 1:
                    return ('_%s_%s' % (word[:-1], word[-1])).lower()
                return '_' + word.lower()
            d['__tablename__'] = _camelcase_re.sub(_join, name).lstrip('_')

        return DeclarativeMeta.__new__(cls, name, bases, d)

    def __init__(self, name, bases, d):
        bind_key = d.pop('__bind_key__', None)
        DeclarativeMeta.__init__(self, name, bases, d)
        if bind_key is not None:
            self.__table__.info['bind_key'] = bind_key


def get_state(app):
    """Gets the state for the application"""
    assert 'sqlalchemy' in app.extensions, \
        'The sqlalchemy extension was not registered to the current ' \
        'application.  Please make sure to call init_app() first.'
    return app.extensions['sqlalchemy']


def signalling_mapper(*args, **kwargs):
    """Replacement for mapper that injects some extra extensions"""
    extensions = to_list(kwargs.pop('extension', None), [])
    extensions.append(_SignalTrackingMapperExtension())
    kwargs['extension'] = extensions
    return sqlalchemy.orm.mapper(*args, **kwargs)


class _SQLAlchemyState(object):
    """Remembers configuration for the (db, app) tuple."""

    def __init__(self, db, app):
        self.db = db
        self.app = app
        self.connectors = {}


class Model(object):
    """Baseclass for custom user models."""

    #: the query class used.  The :attr:`query` attribute is an instance
    #: of this class.  By default a :class:`BaseQuery` is used.
    query_class = BaseQuery

    #: an instance of :attr:`query_class`.  Can be used to query the
    #: database for instances of this model.
    query = None


class SQLAlchemy(object):
    """This class is used to control the SQLAlchemy integration to one
    or more Flask applications.  Depending on how you initialize the
    object it is usable right away or will attach as needed to a
    Flask application.

    There are two usage modes which work very similar.  One is binding
    the instance to a very specific Flask application::

        app = Flask(__name__)
        db = SQLAlchemy(app)

    The second possibility is to create the object once and configure the
    application later to support it::

        db = SQLAlchemy()

        def create_app():
            app = Flask(__name__)
            db.init_app(app)
            return app

    The difference between the two is that in the first case methods like
    :meth:`create_all` and :meth:`drop_all` will work all the time but in
    the second case a :meth:`flask.Flask.request_context` has to exist.

    By default Flask-SQLAlchemy will apply some backend-specific settings
    to improve your experience with them.  As of SQLAlchemy 0.6 SQLAlchemy
    will probe the library for native unicode support.  If it detects
    unicode it will let the library handle that, otherwise do that itself.
    Sometimes this detection can fail in which case you might want to set
    `use_native_unicode` (or the ``SQLALCHEMY_NATIVE_UNICODE`` configuration
    key) to `False`.  Note that the configuration key overrides the
    value you pass to the constructor.

    This class also provides access to all the SQLAlchemy functions and classes
    from the :mod:`sqlalchemy` and :mod:`sqlalchemy.orm` modules.  So you can
    declare models like this::

        class User(db.Model):
            username = db.Column(db.String(80), unique=True)
            pw_hash = db.Column(db.String(80))

    You can still use :mod:`sqlalchemy` and :mod:`sqlalchemy.orm` directly, but
    note that Flask-SQLAlchemy customizations are available only through an
    instance of this :class:`SQLAlchemy` class.  Query classes default to
    :class:`BaseQuery` for `db.Query`, `db.Model.query_class`, and the default
    query_class for `db.relationship` and `db.backref`.  If you use these
    interfaces through :mod:`sqlalchemy` and :mod:`sqlalchemy.orm` directly,
    the default query class will be that of :mod:`sqlalchemy`.

    .. admonition:: Check types carefully

       Don't perform type or `isinstance` checks against `db.Table`, which
       emulates `Table` behavior but is not a class. `db.Table` exposes the
       `Table` interface, but is a function which allows omission of metadata.

    You may also define your own SessionExtension instances as well when
    defining your SQLAlchemy class instance. You may pass your custom instances
    to the `session_extensions` keyword. This can be either a single
    SessionExtension instance, or a list of SessionExtension instances. In the
    following use case we use the VersionedListener from the SQLAlchemy
    versioning examples.::

        from history_meta import VersionedMeta, VersionedListener

        app = Flask(__name__)
        db = SQLAlchemy(app, session_extensions=[VersionedListener()])

        class User(db.Model):
            __metaclass__ = VersionedMeta
            username = db.Column(db.String(80), unique=True)
            pw_hash = db.Column(db.String(80))

    The `session_options` parameter can be used to override session
    options.  If provided it's a dict of parameters passed to the
    session's constructor.

    .. versionadded:: 0.10
       The `session_options` parameter was added.

    .. versionadded:: 0.16
       `scopefunc` is now accepted on `session_options`. It allows specifying
        a custom function which will define the SQLAlchemy session's scoping.
    """

    def __init__(self, app=None, use_native_unicode=True,
                 session_extensions=None, session_options=None):
        self.use_native_unicode = use_native_unicode
        self.session_extensions = to_list(session_extensions, []) + \
                                  [_SignallingSessionExtension()]

        if session_options is None:
            session_options = {}

        session_options.setdefault(
            'scopefunc', connection_stack.__ident_func__
        )

        self.session = self.create_scoped_session(session_options)
        self.Model = self.make_declarative_base()
        self._engine_lock = Lock()

        if app is not None:
            self.app = app
            self.init_app(app)
        else:
            self.app = None

        _include_sqlalchemy(self)
        self.Query = BaseQuery

    @property
    def metadata(self):
        """Returns the metadata"""
        return self.Model.metadata

    def create_scoped_session(self, options=None):
        """Helper factory method that creates a scoped session."""
        if options is None:
            options = {}
        scopefunc=options.pop('scopefunc', None)
        return orm.scoped_session(
            partial(_SignallingSession, self, **options), scopefunc=scopefunc
        )

    def make_declarative_base(self):
        """Creates the declarative base."""
        base = declarative_base(cls=Model, name='Model',
                                mapper=signalling_mapper,
                                metaclass=_BoundDeclarativeMeta)
        base.query = _QueryProperty(self)
        return base

    def init_app(self, app):
        """This callback can be used to initialize an application for the
        use with this database setup.  Never use a database in the context
        of an application not initialized that way or connections will
        leak.
        """
        app.config.setdefault('SQLALCHEMY_DATABASE_URI', 'sqlite://')
        app.config.setdefault('SQLALCHEMY_BINDS', None)
        app.config.setdefault('SQLALCHEMY_NATIVE_UNICODE', None)
        app.config.setdefault('SQLALCHEMY_ECHO', False)
        app.config.setdefault('SQLALCHEMY_RECORD_QUERIES', None)
        app.config.setdefault('SQLALCHEMY_POOL_SIZE', None)
        app.config.setdefault('SQLALCHEMY_POOL_TIMEOUT', None)
        app.config.setdefault('SQLALCHEMY_POOL_RECYCLE', None)

        if not hasattr(app, 'extensions'):
            app.extensions = {}
        app.extensions['sqlalchemy'] = _SQLAlchemyState(self, app)

        # 0.9 and later
        if hasattr(app, 'teardown_appcontext'):
            teardown = app.teardown_appcontext
        # 0.7 to 0.8
        elif hasattr(app, 'teardown_request'):
            teardown = app.teardown_request
        # Older Flask versions
        else:
            teardown = app.after_request

        @teardown
        def shutdown_session(response):
            self.session.remove()
            return response

    def apply_pool_defaults(self, app, options):
        def _setdefault(optionkey, configkey):
            value = app.config[configkey]
            if value is not None:
                options[optionkey] = value
        _setdefault('pool_size', 'SQLALCHEMY_POOL_SIZE')
        _setdefault('pool_timeout', 'SQLALCHEMY_POOL_TIMEOUT')
        _setdefault('pool_recycle', 'SQLALCHEMY_POOL_RECYCLE')

    def apply_driver_hacks(self, app, info, options):
        """This method is called before engine creation and used to inject
        driver specific hacks into the options.  The `options` parameter is
        a dictionary of keyword arguments that will then be used to call
        the :func:`sqlalchemy.create_engine` function.

        The default implementation provides some saner defaults for things
        like pool sizes for MySQL and sqlite.  Also it injects the setting of
        `SQLALCHEMY_NATIVE_UNICODE`.
        """
        if info.drivername == 'mysql':
            info.query.setdefault('charset', 'utf8')
            options.setdefault('pool_size', 10)
            options.setdefault('pool_recycle', 7200)
        elif info.drivername == 'sqlite':
            pool_size = options.get('pool_size')
            detected_in_memory = False
            # we go to memory and the pool size was explicitly set to 0
            # which is fail.  Let the user know that
            if info.database in (None, '', ':memory:'):
                detected_in_memory = True
                if pool_size == 0:
                    raise RuntimeError('SQLite in memory database with an '
                                       'empty queue not possible due to data '
                                       'loss.')
            # if pool size is None or explicitly set to 0 we assume the
            # user did not want a queue for this sqlite connection and
            # hook in the null pool.
            elif not pool_size:
                from sqlalchemy.pool import NullPool
                options['poolclass'] = NullPool

            # if it's not an in memory database we make the path absolute.
            if not detected_in_memory:
                info.database = os.path.join(app.root_path, info.database)

        unu = app.config['SQLALCHEMY_NATIVE_UNICODE']
        if unu is None:
            unu = self.use_native_unicode
        if not unu:
            options['use_native_unicode'] = False

    @property
    def engine(self):
        """Gives access to the engine.  If the database configuration is bound
        to a specific application (initialized with an application) this will
        always return a database connection.  If however the current application
        is used this might raise a :exc:`RuntimeError` if no application is
        active at the moment.
        """
        return self.get_engine(self.get_app())

    def make_connector(self, app, bind=None):
        """Creates the connector for a given state and bind."""
        return _EngineConnector(self, app, bind)

    def get_engine(self, app, bind=None):
        """Returns a specific engine.

        .. versionadded:: 0.12
        """
        with self._engine_lock:
            state = get_state(app)
            connector = state.connectors.get(bind)
            if connector is None:
                connector = self.make_connector(app, bind)
                state.connectors[bind] = connector
            return connector.get_engine()

    def get_app(self, reference_app=None):
        """Helper method that implements the logic to look up an application.
        """
        if reference_app is not None:
            return reference_app
        if self.app is not None:
            return self.app
        ctx = connection_stack.top
        if ctx is not None:
            return ctx.app
        raise RuntimeError('application not registered on db '
                           'instance and no application bound '
                           'to current context')

    def get_tables_for_bind(self, bind=None):
        """Returns a list of all tables relevant for a bind."""
        result = []
        for table in self.Model.metadata.tables.itervalues():
            if table.info.get('bind_key') == bind:
                result.append(table)
        return result

    def get_binds(self, app=None):
        """Returns a dictionary with a table->engine mapping.

        This is suitable for use of sessionmaker(binds=db.get_binds(app)).
        """
        app = self.get_app(app)
        binds = [None] + list(app.config.get('SQLALCHEMY_BINDS') or ())
        retval = {}
        for bind in binds:
            engine = self.get_engine(app, bind)
            tables = self.get_tables_for_bind(bind)
            retval.update(dict((table, engine) for table in tables))
        return retval

    def _execute_for_all_tables(self, app, bind, operation):
        app = self.get_app(app)

        if bind == '__all__':
            binds = [None] + list(app.config.get('SQLALCHEMY_BINDS') or ())
        elif isinstance(bind, basestring):
            binds = [bind]
        else:
            binds = bind

        for bind in binds:
            tables = self.get_tables_for_bind(bind)
            op = getattr(self.Model.metadata, operation)
            op(bind=self.get_engine(app, bind), tables=tables)

    def create_all(self, bind='__all__', app=None):
        """Creates all tables.

        .. versionchanged:: 0.12
           Parameters were added
        """
        self._execute_for_all_tables(app, bind, 'create_all')

    def drop_all(self, bind='__all__', app=None):
        """Drops all tables.

        .. versionchanged:: 0.12
           Parameters were added
        """
        self._execute_for_all_tables(app, bind, 'drop_all')

    def reflect(self, bind='__all__', app=None):
        """Reflects tables from the database.

        .. versionchanged:: 0.12
           Parameters were added
        """
        self._execute_for_all_tables(app, bind, 'reflect')

    def __repr__(self):
        app = None
        if self.app is not None:
            app = self.app
        else:
            ctx = connection_stack.top
            if ctx is not None:
                app = ctx.app
        return '<%s engine=%r>' % (
            self.__class__.__name__,
            app and app.config['SQLALCHEMY_DATABASE_URI'] or None
        )