Flask-SQLAlchemy - separating reads and writes

config.py

# This is not used unless SQLALCHEMY_BINDS is not present
SQLALCHEMY_DATABASE_URI = 'mysql+pymysql://{username}:{password}@{hostname}/{database}?charset=utf8'

SQLALCHEMY_BINDS = {
  'master': 'mysql+pymysql://{username}:{password}@{hostname}/{database}?charset=utf8',
  'slave': 'mysql+pymysql://{username}:{password}@{hostname}/{database}?charset=utf8'
}

routing.py

from flask.ext.sqlalchemy import  SQLAlchemy, get_state
import sqlalchemy.orm as orm
from functools import partial

import logging

log = logging.getLogger(__name__)

class AutoRouteSession(orm.Session):

  def __init__(self, db, autocommit=False, autoflush=False, **options):
    self.app = db.get_app()
    self._model_changes = {}
    orm.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=None, clause=None):

    try:
      state = get_state(self.app)
    except (AssertionError, AttributeError, TypeError) as err:
      log.info("Unable to get Flask-SQLAlchemy configuration. Outputting default bind. Error:" + err)
      return orm.Session.get_bind(self, mapper, clause)

    # If there are no binds configured, connect using the default SQLALCHEMY_DATABASE_URI
    if state is None or not self.app.config['SQLALCHEMY_BINDS']:
      if not self.app.debug:
        log.debug("Connecting -> DEFAULT. Unable to get Flask-SQLAlchemy bind configuration. Outputting default bind." )
      return orm.Session.get_bind(self, mapper, clause)

    # Writes go to the master
    elif self._flushing: # we who are about to write, salute you
      log.debug("Connecting -> MASTER")
      return state.db.get_engine(self.app, bind='master')

    # Everything else goes to the slave
    else:
      log.debug("Connecting -> SLAVE")
      return state.db.get_engine(self.app, bind='slave')

class AutoRouteSQLAlchemy(SQLAlchemy):

    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(AutoRouteSession, self, **options), scopefunc=scopefunc
      )

@andyxning

Getting the error TypeError: make_declarative_base() takes at least 2 arguments (1 given) when I use that code.

How can I integrate this with a sessionmaker (currently i pass an engine to the session manager in the begging)?

engine = SessionManager.get_engine(uri)
session = sessionmaker(bind=engine)()

I tried to make the session object like this:

db = AutoRouteSQLAlchemy()
session = db.session

but i got:

AttributeError: 'scoped_session' object has no attribute '_autoflush'

later on in the code