Re: multi-table inheritance and modification of OneToOneField options

[Malcolm Tredinnick]

On Fri, 2009-08-07 at 00:30 -0700, Jan Ostrochovsky wrote:
> Thank you, Malcolm.
> 
> In fact, our current design was, as you recommended:
> 
> Subject ---one-to-one-null-false--- Address ---one-to-one-null-true---
> Payment (Invoice)
> 
> I want to use generic views and to join Subject+Address in the same
> form, and model inheritance seemed to me as most straightforward
> solution for this need, but as you do not recommend it,

Bear in mind that there might be somebody who gives stricter
recommendations on not abusing model inheritance around here, but I'm
not sure who they would be. I very much view it as a Python equivalent
(with some abstraction leakage from DB -> Python), but my view isn't
universally held. In this case, though, what you're asking for isn't
possible in the way the code is implemented, so it's not just a matter
of preference.

>  I will look
> for another solution... e.g. form inheritance, as I found it here
> http://docs.djangoproject.com/en/dev/topics/forms/modelforms/#form-inheritance
> or here http://www.djangosnippets.org/snippets/703/.

Hmm ... if you're doing this for the form result and you aren't
intermixing the fields from Subject and Address (or even if you are),
you might be better served by still making each on its own ModelForm and
just passing both into the template. Then layout both Django forms
inside a single HTML form element.

Regards,
Malcolm



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Re: multi-table inheritance and modification of OneToOneField options

[Jan Ostrochovsky]

Thank you, Malcolm.

In fact, our current design was, as you recommended:

Subject ---one-to-one-null-false--- Address ---one-to-one-null-true---
Payment (Invoice)

I want to use generic views and to join Subject+Address in the same
form, and model inheritance seemed to me as most straightforward
solution for this need, but as you do not recommend it, I will look
for another solution... e.g. form inheritance, as I found it here
http://docs.djangoproject.com/en/dev/topics/forms/modelforms/#form-inheritance
or here http://www.djangosnippets.org/snippets/703/.

Jano

On Aug 7, 3:50 am, Malcolm Tredinnick 
wrote:
> On Thu, 2009-08-06 at 07:15 -0700, Jan Ostrochovsky wrote:
> > Hello,
>
> >http://docs.djangoproject.com/en/dev/topics/db/models/#multi-table-in...
> > says:
> > The inheritance relationship introduces links between the child model
> > and each of its parents (via an automatically-created OneToOneField).
>
> >http://docs.djangoproject.com/en/dev/ref/models/fields/#onetoonefield
> > says: Multi-table inheritance is implemented by adding an implicit one-
> > to-one relation from the child model to the parent model...
>
> > The question is: how can I set options of that implicit OneToOneField?
> > Most interesting is null=True / null=False.
>
> You don't. Model inheritance is a Python-level thing and things like
> "optional" inheritance doesn't existence for Python inheritance. It's a
> shortcut for emulating Python class inheritance as much as possible.
>
> If you want this level of control, model the relations explicitly and
> set the options on the OneToOneField to whatever you like.
>
> Regards,
> Malcolm
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Re: multi-table inheritance and modification of OneToOneField options

[Malcolm Tredinnick]

On Thu, 2009-08-06 at 07:15 -0700, Jan Ostrochovsky wrote:
> Hello,
> 
> http://docs.djangoproject.com/en/dev/topics/db/models/#multi-table-inheritance
> says:
> The inheritance relationship introduces links between the child model
> and each of its parents (via an automatically-created OneToOneField).
> 
> http://docs.djangoproject.com/en/dev/ref/models/fields/#onetoonefield
> says: Multi-table inheritance is implemented by adding an implicit one-
> to-one relation from the child model to the parent model...
> 
> The question is: how can I set options of that implicit OneToOneField?
> Most interesting is null=True / null=False.

You don't. Model inheritance is a Python-level thing and things like
"optional" inheritance doesn't existence for Python inheritance. It's a
shortcut for emulating Python class inheritance as much as possible.

If you want this level of control, model the relations explicitly and
set the options on the OneToOneField to whatever you like.

Regards,
Malcolm


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multi-table inheritance and modification of OneToOneField options

[Jan Ostrochovsky]

Hello,

http://docs.djangoproject.com/en/dev/topics/db/models/#multi-table-inheritance
says:
The inheritance relationship introduces links between the child model
and each of its parents (via an automatically-created OneToOneField).

http://docs.djangoproject.com/en/dev/ref/models/fields/#onetoonefield
says: Multi-table inheritance is implemented by adding an implicit one-
to-one relation from the child model to the parent model...

The question is: how can I set options of that implicit OneToOneField?
Most interesting is null=True / null=False.

(I want to "inherit" to one class optionally (Invoice Address) and to
one class it is required (Residence Address). Base class is Address.)

Thanks in advance.

Jan Ostrochovsky

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