[Grok-dev] Modeling a class that extend from list() with megrok.rdb

Hector Blanco white.lists at gmail.com
Wed Oct 27 15:31:23 EDT 2010

2010/10/27 Jeffrey D Peterson <bgpete at gmail.com>:
> I am a little confused to what you are trying to accomplish.

I know... I understand. I'm sorry I can't explain myself better... But
as I realized by reading your reply, you got it perfectly right :-)

> Maybe it would be better to define what you need it to do, and model that
> vs. trying to equivocate things class by class, I realize you want to save
> code but you are converting from object data to relational data, they aren't
> always converted easily.

Yeah, I know... "If it can't be done, it can't be done."

> Extrapolating from what you have shown us it seems you are storing MyEntry
> objects in extended python lists, correct?  I assume, that these lists are
> stored in the ZODB for persistence.


> Now with relational data you essentially get back a list of dictionaries,
> right?  Rows of key-value pairs.  With SQLAlchemy you can change that a
> little to where you wind up with essentially a list of objects, where each
> row is an object and the key-value pairs are properties of the object.

Yes. Actually, I got that working that way in some other places, where
the "container" was just a regular python list() object.

Like... I don't know...

whatever_to_items = Table(
	Column("whatever_container_id", Integer, ForeignKey("whatever_containers.id")),
	Column("item_id", Integer, ForeignKey("items.id"))

class WhateverContainer(rdb.Model):
	items = relationship("Item", secondary=whatever_to_items,

class Item(rdb.Model):
	"""Set up items table in the database"""

	id = Column("id", Integer, primary_key=True)

That works fine.  The thing is that in this case, the relationship
needs to be handled by one of my classes (The MyContainer() class)
which, if I understood properly, also needs to be stored in the
database (and therefore it needs to extend from rdb.Model and having
its own id, right?) And that extension from rdb.Model is what messes
up with my class (which, until the moment, it was happily) extending
from list()

> In each case, your "entries" are contained in a list.  With SQLAlchemy you
> are closer as it's a list of objects, _and_ you can write methods easily to
> access the objects.
> If it needs to be sorted out into multiple named containers, then you need
> to add that wrinkle, by adding a container id to the entry object and either
> managing that by hand or by maintaining a table of container IDs
> So either you'll end up with a query that looks at a container table and
> joins the entries or you'll have an entries table that you can query for
> only certain entries based on a container ID
> This is what I think you are getting at, am I even close?

Yes, you're perfectly right. As I mentioned, I have successfully modeled
raw "lists" as relationships with foreign tables. This one is
problematic because it is not exactly a list() (in the Python
"sense"). It is a list with a bunch of special methods. That's what
I'd like to keep but, if it can't be done, it can't be done.

Maybe I will try the:

class MyContainer(rdb.Model):
       def __init__(self):
              self._list = list()
I'm sure that will save me a lot of headaches...

Thanks for not giving up! :-) I'll keep you posted.

> --
> Jeffrey Peterson
> bgpete3 at gmail.com
>> -----Original Message-----
>> From: grok-dev-bounces at zope.org [mailto:grok-dev-bounces at zope.org] On
>> Behalf Of Hector Blanco
>> Sent: Wednesday, October 27, 2010 9:06 AM
>> Cc: grok-dev at zope.org
>> Subject: Re: [Grok-dev] Modeling a class that extend from list() with
>> megrok.rdb
>> Oh, now I see what you meant. Thanks!
>> But there's still a problem (sorry, I'm really new to this and I may
>> have the concepts wrong):
>> The class I want it to attack the "my_containers_table" table is
>>     class MyContainer(list)
>> which currently extends from list (not rdb.Model).
>> Is there a way to have both functionalities? Do I have to go to
>> multiple inheritance? ( class MyContainer(list, rdb.Model)   ).
>> Wouldn't that be "dangerous"? (I have been thought to avoid multiple
>> inheritance as the plague).
>> Do I have to do something like:
>>    class MyContainer(rdb.Model):
>>         def __init__(self):
>>                self._list = list()
>> and use that self._list to store the data?
>> Thank you for your help!
>> 2010/10/26 Jeffrey D Peterson <bgpete at gmail.com>:
>> >
>> >
>> > --
>> > Jeffrey Peterson
>> > bgpete3 at gmail.com
>> >

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