On Sun, Jul 23, 2017 at 11:26 AM, Ruben Di Battista
<rubendibatti...@gmail.com> wrote:
> Hello,
>
> I'm trying to introduce database persistence into an already existent class
> hierarchy. That mean I have in a separate module all the class representing
> the models of my application (without SQL interaction methods), then I have
> in another module the SQL schema and the mapping (so I'm using the classical
> mapping):
>
> sql.py
>
> import sqlalchemy.orm as orm
> from sqlalchemy import Metadata, Integer, String, DateTime
> from myproject.models import Item, Order
>
> metadata = MetaData()
>
> item = Table('item', metadata,
> Column('id', Integer, primary_key=True),
> Column('name', String),
> Column('quantity', Integer)
> )
>
> order = Order('order', metadata,
> Column('id', Integer, primary_key=True),
> Column('date', DateTime),
> )
>
> orm.mapper(Item, item)
> orm.mapper(Order, order)
>
>
>
>
> models.py
> class Item(object):
> def __init__(self, name, quantity):
> self.name = name
> self.quantity = quantity
>
> class Order(object):
> def __init__(self, date):
> self.date = date
>
> Now I would like to introduce a M2M relationship with additional column in
> the association table:
>
> order_item = Table('order_item', metadata,
> Column('order_id', Integer, ForeignKey('order.id')),
> Column('item_id', Integer, ForeignKey('item.id')),
> Column('price', Float)
> )
>
>
> In the documentation, when they present the Association proxy, they do that
> in a declarative way. So they add an attribute to the class:
>
> # ... Class definition ...
> items = association_proxy('order_items', 'items')
>
>
> Is there a way to achieve this without touching the classes in models.py?
> Maybe using the orm.mapper? Or the only way is to modify the class adding
> the association proxy like in the documentation? Do you have any reference
> to point me out on the right direction to use association proxies with
> classical mapping?
Basically, just do this:
orm.mapper(Order, order)
Order.items = association_proxy('order_items', 'items')
The association proxy is independent of the declarative system and if
you look in early documentation you'll see it described in terms of
classical mapping.
As far as the term "without touching the classes", there's not much
point trying to be purist about this as the call to mapper(Order,
order) highly alters the Order class in any case.
>
> Thanks,
>
> PS: You find this same question on SO, if you want you can answer also
> there.
>
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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