[Chicago] help from Django and Pylons developers

Kumar McMillan kumar.mcmillan at gmail.com
Mon Apr 28 22:15:39 CEST 2008


On Mon, Apr 28, 2008 at 1:18 PM, Ian Bicking <ianb at colorstudy.com> wrote:
> > I have never a Pylons example for connecting to multiple databases. Would
> you mind posting an example?
> >

I can't find this documented anywhere but if you configure a Pylons
app with this structure (using SQLAlchemy 0.4),
http://wiki.pylonshq.com/display/pylonsdocs/Using+SQLAlchemy+with+Pylons

... then, if you wanted to, say, connect to a read-only db and a
write-only db, you could update your config like this:

[app:main]
# ...
read_db.url = postgres://read_user:pw@readhost/read_db
#read_db.debug = True
write_db.url = postgres://write_user:pw@writehost/write_db

... and you could change environment.py to call init_model() like:

def load_environment(global_conf, app_conf):
    # ...
    read_engine = engine_from_config(config, 'read_db.')
    write_engine = engine_from_config(config, 'write_db.')
    init_model(read_engine, write_engine)

... and update the init_model() def in model/__init__.py to :

from sqlalchemy import orm
from myapp.model import meta

def init_model(read_engine, write_engine):
    ReadSession = orm.scoped_session(orm.sessionmaker(autoflush=False,
transactional=False, bind=read_engine))
    meta.read_metadata.bind = read_engine
    meta.read_session = ReadSession()
    # setup read mappers here..

    WriteSession = orm.scoped_session(orm.sessionmaker(autoflush=True,
transactional=True, bind=write_engine))
    meta.write_metadata.bind = write_engine
    meta.write_session = WriteSession()
    # setup write mappers here..


In other words, the config file is just a place to store values, any
values you want, and the environment.py module gives you the chance to
customize the initialization of your app any way you want.  You store
the initialization code wherever you want, Pylons doesn't force any
conventions on you other than by creating a default module layout when
you start your project.

Kumar


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