see (single) table inheritance and the rest,
http://www.sqlalchemy.org/docs/05/mappers.html#advdatamapping_mapper_inheritance
On Wednesday 03 September 2008 21:59:28 [EMAIL PROTECTED] wrote:
> 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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