[plt-scheme] Resources in the transformer environment

On 10/26/06, Keith Frost <keith.l.frost at gmail.com> wrote:
> I'm not at all sure that macro-expansion time is the best time to be
> making a connection to a database.

I agree, but the PLT class/object system makes it impossible to do at
runtime (as far as I can tell).

> It might be better -- and trivial
> in scheme -- to implement a separate program which walks the code,
> makes any database connections and sets up any necessary schema, and
> then leaves some record of its having done so, to avoid unnecessary
> repetition.  When the component is then loaded into the web-server
> process, it could check to see if things were setup properly, and do
> something appropriate if not.
>

Minor issue: I'm not setting up the schema from the code, but the
reverse.  Not that it matters.  You're right; I could easily do this
with a separate program.  I'd prefer not to, since I'd rather not have
to run this program after every schema change, but that's not too
onerous.

-Jon




> -- Keith F.
>
> On 10/26/06, Jon Zeppieri <zeppieri at gmail.com> wrote:
> > I've been playing around with class.ss, attempting to implement
> > something similar to Ruby's ActiveRecord class, which provides an
> > automatic mapping between a class and a relational database table.
> >
> > I have a rudimentary form of this working, where each column from the
> > DB table is represented by a field in the class.  This is accomplished
> > by a macro, dbo-class*, which expands to a class* form.  Since the DB
> > columns have to be known at expansion time (and since a goal of this
> > project is to create the class automatically from the DB schema), I
> > need to connect to the database during macro expansion.  However,
> > using instances of the created classes obviously requires a connection
> > to the database during normal runtime.
> >
> > As far as I can tell, there is no way to do this without creating two
> > different connections, since the transformer and runtime environments
> > are kept completely separate.  Now, opening two database connections
> > isn't such a problem, but I am bothered by the fact that while I can
> > sensibly manage the runtime connection (e.g., I can use dynamic-wind
> > to create, use, and close the connection reliably), I'm not sure how
> > to manage the connection in the transformer environment.
> >
> > Below is an example of the class's use.  Some information that might
> > be useful in making sense of it:
> >   - current-connection is a parameter provided my mysql.ss
> >   - dbo.ss provides the dbo-class* macro
> >   - the first parameter to dbo-class* is the name of the database
> > table to be represented by the class
> >  - the find method returns instances of the class, each of which
> > represents a row of the DB table
> >
> > ==============================
> > (require-for-syntax (lib "mysql.ss" "mysql"))
> > (begin-for-syntax (current-connection (connect "host.domain.foo" 3306
> > "my_username" "my_password" "my_schema")))
> >
> > (require (lib "mysql.ss" "mysql"))
> > (require (lib "dbo.ss" "dbo"))
> >
> > (dynamic-wind
> >  (lambda ()
> >    (current-connection ()connect "host.domain.foo" 3306 "my_username"
> > "my_password" "my_schema"))
> >  (lambda ()
> >    (define project%
> >      (dbo-class* "project" object% ()
> >                  (super-new)))
> >
> >    (define o (make-object project%))
> >    (define ps (send o find #:all))
> >    (display ps))
> >  (lambda ()
> >    (close (current-connection))))
> >
> > (begin-for-syntax (close (current-connection)))
> >
> > ===========================
> >
> > The above code works fine, but I'd like to use the dbo classes in web
> > servlets, where I would be a lot more concerned about the lifetime of
> > database connections.  That final (begin-for-syntax) in the above
> > example doesn't seem to belong anywhere in a servlet module.
> >
> > Any ideas?  If it would be at all useful to see the dbo.ss code (or
> > anything else), I can send it along.
> >
> > -Jon
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