On Aug 17, 5:40 am, "King Simon-NFHD78" <[email protected]>
wrote:
> > -----Original Message-----
> > From: [email protected]
> > [mailto:[email protected]] On Behalf Of darkblueB
> > Sent: 17 August 2009 06:31
> > To: sqlalchemy
> > Subject: [sqlalchemy] new questions
>
> > Hi-
> > I have just read a lot and gone through some exercises, but am at an
> > early stage here..
> > Two questions:
>
> > 1) I have an existing database, and want to use sa to work with it..
> > so "reflect" seems like what I want. I have loaded up the database
> > successfully - sqlite - not with reflect(), but instead with named
> > tables and autoload=true.
>
> > Now I am reading about declarative base class.. is a reflect/
> > autoload approach preemptive of declarative? do I have to make
> > mappers? there are a number of foreign key relationships, declared in
> > the db.. how do I find out more about which ones are being
> > instantiated for me?
>
> > 2) Session vs connect.. from my limited experience, conn =
> > engine.connect() seems natural and all I need. I am reading this intro
> > material and they are going on at length with Session. Is Session
> > really necessary? so much more desirable?
>
> > I am really looking for an expedient use of sa, I dont think I will
> > get deeply into this.. pragmatic!
> > thanks in advance
> > -Brian
>
> Hi,
>
> In answer to your first question, the declarative extension is pretty
> flexible. You can still use autoload to get the table definitions, and
> then explicitly add anything that the autoload doesn't pick up. There is
> an example
> athttp://www.sqlalchemy.org/docs/05/reference/ext/declarative.html#table-c
> onfiguration
>
> Once your declarative class has been created, you can access the
> underlying table object as "YourMappedClass.__table__". This table
> should have a foreign_keys property which lists all the forgein keys, so
> you can see whether they have been detected correctly.
>
> As for your second question, this is mostly about the difference between
> the "SQL Expression Language" layer of SQLAlchemy, which generally works
> with engines and connections, and the "ORM" layer, which normally works
> with the Session. The description
> athttp://www.sqlalchemy.org/docs/05/session.html#what-does-the-session-do
> is probably a reasonable description of why you might want to use a
> Session.
>
> If you are only ever reading data from the database, and you don't want
> or need to build object relationships on top of your SQL data, you can
> easily get away with the SQL Expression Language layer and not need the
> ORM. However, if you need to do much more than basic manipulation of the
> data, the ORM can make your life much easier.
>
> Hope that helps,
>
> Simon
Hi Simon
thanks for the reply.. Your second part is straightforward..
The first one, not so much..
I have gone back to a simple
meta.bind = //my engine here
meta.reflect()
I can see meta.tables, and meta.tables_sorted()
but how do I know what mapped objects exist?
(there should be about 12 tables, with a few one to many and one many
to many defined)
I feel like time is passing by, I would like to use ORM but this is
new to me
thanks much
-Brian
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