> -----Original Message-----
> From: sqlalchemy@googlegroups.com
> [mailto:sqlalch...@googlegroups.com] On Behalf Of
> thatsanicehatyouh...@mac.com
> Sent: 07 July 2010 20:33
> To: sqlalchemy@googlegroups.com
> Subject: Re: [sqlalchemy] Using the declarative base across projects
>
> Hi Lance,
>
> Thanks for your comments.
>
> On Jul 7, 2010, at 12:28 PM, Lance Edgar wrote:
>
> > Why not just do this in project2 ?
> >
> >
> > import project.DatabaseConnection as db
> >
> > Base = declarative_base(bind=db.engine)
> >
> > # ... etc.
>
> The DatabaseConnection class contains the particulars of the
> connection (i.e. host, username, password) which can be
> different. I don't want to create dependencies between the
> projects, I just want to reuse the class definitions. I want
> to create the DatabaseConnection once and "pass" it into the
> definition of the classes.
>
> Another approach I tried was to make ModelClasses an object
> and define the classes in a method there (so I could just
> pass the Base class to it), but the class definitions were in
> the wrong namespace.
>
> >> The "python way" seems to be to create a "config" class,
> but project2.ModelClasses won't know anything about it if
> it's defined in the first project. As to the reason why there
> are two separate projects, consider the case where one set of
> tables is one logical group, and the second is a replicated
> copy from another server. I can't merge all of these projects
> since they really are independent "units", but sometimes I
> will link them (as above).
> >
> > I don't understand why project2 wouldn't "know anything
> about it" if defined in (first) project. All it needs to do
> is import the connection info from the project (as in above
> example). If the database configuration really transcends
> both project and project2 though, then yes it probably could
> be wrapped in a config module of some sort in another
> project; depending on the scope that may be a bit overkill.
> If you can consider either "project" or "project2" to be
> slightly more default than the other then the db config could
> stay there I'd think.
>
> This is a bit tricky to explain. Imagine I have one database,
> and I create a project (1) to work with that database
> (connections, table class definitions, etc.). That is
> standalone (to me). I have another completely separate
> database (2) on another host where I do the same thing. Using
> replication I then create a read-only copy of database 1 in
> database 2, and join some of the tables. Project 2 needs to
> generate the classes, but use SA's Base class that is
> dynamically generated. Since it's dynamic, I have to create
> it at run time... but now I can't pass that to the definition
> of project 1's classes. It's that communication that I'm
> struggling with.
>
> Cheers,
> Demitri
>
In general, you don't need a database connection just to define your
tables and mappers. The 'bind' parameter to DeclarativeBase is optional,
and only necessary if you are using autoloading. So one solution to your
problem would be not to use autoloading, and bind to a database at the
Session level rather than the Mapper level. That would be the usual way
to use the same set of classes against multiple databases.
If you really need to use autoloading, you could move all your class
definitions into a function that accepts a database engine as a
parameter. For example:
#################
# ModelClasses.py
class Namespace(object):
def __init__(self, **kwargs):
self.__dict__.update(kwargs)
def initdb(connection_string):
engine = create_engine(connection_string)
Base = declarative_base(bind=engine)
class Table1(Base):
__tablename__ = 'table1'
__table_args__ = {'autoload': True}
return Namespace(Base=Base,
Table1=Table1)
# or, you could be lazy:
# return Namespace(**locals())
####################
# MainScript1.py
import ModelClasses
db = ModelClasses.initdb(my_connection_string)
# access db.Table1, db.Base etc.
Hope that helps,
Simon
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