Turbogears2 Change The Way Your App Loads The Database Engines

Step by Step
1. In configuration file (development.ini). I change sqlalchemy

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#sqlalchemy.url = sqlite:///%(here)s/devdata.db
sqlalchemy.first.url = sqlite:///%(here)s/database_1.db
sqlalchemy.second.url = sqlite:///%(here)s/database_2.db

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2. In Application Configuration to load the multiple databases. In file ../config/app_cfg.py

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# make sure these imports are added to the top
from tg.configuration import AppConfig, config
from myapp.model import init_model

# add this before base_config =
class MultiDBAppConfig(AppConfig): 

    def _setup_sqlalchemy(self, conf):

    #def setup_sqlalchemy(self):
        """Setup SQLAlchemy database engine(s)"""
        from sqlalchemy import engine_from_config
        engine1 = engine_from_config(config, 'sqlalchemy.first.')
        engine2 = engine_from_config(config, 'sqlalchemy.second.')
        # engine1 should be assigned to sa_engine as well as your first engine's name
        config['tg.app_globals'].sa_engine = engine1
        config['tg.app_globals'].sa_engine_first = engine1
        config['tg.app_globals'].sa_engine_second = engine2
        # Pass the engines to init_model, to be able to introspect tables
        init_model(engine1, engine2)

#base_config = AppConfig()
base_config = MultiDBAppConfig()

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In document Turbogears2 it used def setup_sqlalchemy(self): , when server start will error.
change to def _setup_sqlalchemy(self, conf): ,  server start will not error.

3. Update your model's __init__ to handle multiple Sessions and Metadata. Add maker2, DBSession2, DeclarativeBase2 and metadata2. Change function init_model in file ../model/__init__.py

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# after the first maker/DBSession assignment, add a 2nd one
maker2 = sessionmaker(autoflush=True, autocommit=False,
                     extension=ZopeTransactionExtension())
DBSession2 = scoped_session(maker2)

# after the first DeclarativeBase assignment, add a 2nd one
DeclarativeBase2 = declarative_base()

# uncomment the metadata2 line and assign it to DeclarativeBase2.metadata
metadata2 = DeclarativeBase2.metadata

# finally, modify the init_model method to allow both engines to be passed (see previous step)
# and assign the sessions and metadata to each engine
def init_model(engine1, engine2):
    """Call me before using any of the tables or classes in the model."""

    #DBSession.configure(bind=engine)
    DBSession.configure(bind=engine1)
    DBSession2.configure(bind=engine2)

    metadata.bind = engine1
    metadata2.bind = engine2

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4. Tell Your Models Which Engine To Use
     4.1 use engine1. create file ../model/spam.py . when query data use DBSession for database_1.db.

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from sqlalchemy import Table, ForeignKey, Column
from sqlalchemy.types import Integer, Unicode, Boolean
from myapp.model import DeclarativeBase, DBSession

class Spam(DeclarativeBase):
    __tablename__ = 'spam'

    def __init__(self, id, variety):
        self.id = id
        self.variety = variety

    id = Column(Integer, autoincrement=True, primary_key=True)
    variety = Column(Unicode(50), nullable=False)

    @classmethod
    def selectAll(cls):
        return DBSession.query(cls).all(); 

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     4.2 use engine2. create file ../model/eggs.py. when query data use DBSession2 for database_2.db.

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from sqlalchemy import Table, ForeignKey, Column
from sqlalchemy.types import Integer, Unicode, Boolean
from myapp.model import DeclarativeBase2, DBSession2

class Eggs(DeclarativeBase2):
    __tablename__ = 'eggs'

    def __init__(self, id, pkg_qty):
        self.id = id
        self.pkg_qty = pkg_qty

    id = Column(Integer, autoincrement=True, primary_key=True)
    pkg_qty = Column(Integer, default=12)

    @classmethod
    def selectAll(cls):
        return DBSession2.query(cls).all();  

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I read at http://turbogears.readthedocs.io/en/latest/cookbook/multiple-databases.html#change-the-way-your-app-loads-the-database-engines