[Chandler-dev] [Sum] The Great Architecture Discussion of 2007

[Mikeal Rogers]

On Oct 9, 2007, at 6:58 PM, Phillip J. Eby wrote:

> Now, Hibernate is not available for Python (although I suppose you  
> could make it so with JCC!) but it illustrates the point that is  
> possible to separate things in this fashion.  I believe there is at  
> least one Python ORM that claims to be inspired by or to work like  
> Hibernate, though.  I also seem to recall that SQLAlchemy for  
> Python also has a great deal of flexibility in mapping between  
> different relational schemas, such that your code can deal with a  
> logical schema rather than an actual one.
>
> There is also the possibility of just rolling Yet Another Python  
> ORM, perhaps based on EIM.  But these things don't matte as much as  
> layering the application in such a way that it does not *care* how  
> things actually get stored.  Chandler's domain model objects should  
> not be subclasses of a storage type, for example.  (i.e., they  
> should not be repository.Items).

I've spent a lot of time with Python ORM's so I thought I'd chime in  
about a few things.

The primary ORM's I know of in python are SQLObject, SQLAlchemy ORM  
and Django ORM.

Django ORM has a lot of extra web specific functionality in it that  
we don't need or want.

SQLObject has a very easy to use API but customizing it's queries is  
somewhere between difficult and impossible. I've seen some people  
complain about scaling SQLObject but haven't used it enough to  
comment on that myself.

TurboGears and Pylons both used to default to using SQLObject but  
have now transitioned to SQLAlchemy. There is also some work in the  
Django community to move the Django ORM over to using SQLAlchemy for  
it's underlying database logic.

SQLAlchemy is a full featured db library which includes a SQL  
expression language ( kind of like Hibernates HQL ), session  
management, various db engines, and an ORM. The vanilla Alchemy ORM  
is very verbose to use which has led a lot of people to design more  
declarative libraries which use the alchemy ORM beneath. The most  
popular right now is Elixir, http://elixir.ematia.de/ .

It's important to note that the alchemy ORM doesn't give you any way  
to define custom marshalling or validation, that all has to be done  
at a layer above the Alchemy ORM. Instances of Elixir classes are  
just instances of Alchemy ORM classes, they don't try to do anything  
in their area.

I've actually put a fair amount of time in to a layer above Alchemy's  
ORM that provides you with custom field classes that you can use to  
define your own marshalling and validation, simpler relationship  
defintions, and a declarative layer that's as easy to use as  
SQLObjects. Hopefully that will get released some time this month, I  
meant to finish it weeks ago but various other projects got in the  
way. I'll make sure to ping this list once I'm done.

-Mikeal

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