Hi all,
I just started playing with SQLAlchemy today (after several years of
plain SQL experience) and I must say I'm impressed. I'm reading my way
through the docs now, but there is one thing I can't seem to find. Let
me briefly explain the situation.
I was given the task of rewriting a database which is in use for many
years now. And since many applications depend on its current structure
I can only make small changes at the time. My plan is to rewrite all
the attached applications but this time abstracting the app's logic
from the data-structure itself. I think that SQLAlchemy will allow me
to achieve this task by building a library of POPO's and some mappers
to the data-structure. In that way I can rework the database and only
have to adapt the mappers to keep my app's running. So I started that
and immediately stumbled upon a 'common' situation which I don't now
how to solve in SQLA. So here goes:
I have 1 table (mytable) which is structured somewhat like this:
id = int (primary key)
name = varchar()
type = int
Now all rows with a type, say 1 'constitute' a MyObject. And rows with
type say 2 are MyOtherObject instances, and so on. So in my
applications I want to create a class like this:
class MyObject(object):
def __init__(self, name):
self.name = name
Then I need to map this to the database. So I write a mapper like
this:
myobject_table = select([mytable], mytable.c.type ==
1).alias('somealias') (not sure if this is entirely correct. I'm
writing this post at home and don't have access to my code at the
office. But this is not the point so...)
mapper(MyObject, myobject_table)
So far all ok, but now when I insert new instances of type MyObject,
the type column is not filled with value 1. The instance is inserted
ok except for this 'hidden' column. I don't want to add this column to
my MyObject class since I foresee that the structure of my DB will
change and then there will be no more value for the type column. The
column 'type' belongs to the internals of my data-structure and
shouldn't be visible in my app's. In the new structure there will be a
table just for MyObject instances.
Does any guru out there knows how to solve this rather 'common'
problem?
Many thanks for reading this post!
--
Wim
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