Re: How to establish foreign key contraints across models living in different applications

[David De La Harpe Golden]

guillermooo wrote:

> 
> I suppose you mean in terms of performance? Could you please elaborate
> on the disadvantages?
> 

They're not database-level foreign keys.  So no db-level
integrity checks. You also can't presently use them in an easy manner in
more complex queries (aggregation).  They might be a bit nonobvious to
someone looking at the RDBMS level, not exactly typical db design.

For some applications, you might also find they're "not generic enough"
- django doesn't _require_ primary keys to be integers, but it tends to
get assumed a lot - and each generic relation can only be generic across
keys of one type.

Really, I'd expect a lot of the time there could just be a modelling
issue that could be sorted by a rethink.  I started trying to use them
for a row-level permissioning scheme, but it ended up easier to just
use a foreign key field to a security descriptor table on every model I
wanted to use row level permissioning for (turns out setting row-level
permissions on other apps' models tends not to be very useful since
those apps won't be written to support them...)
















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Re: How to establish foreign key contraints across models living in different applications

[guillermooo]

Paul,

> then generic keys may be the way to go.  If however, you are simply
> having one of your own apps depend on another I strongly suggest not
> using generic foreign keys, they have drawbacks that could hinder you
> down the road.

I suppose you mean in terms of performance? Could you please elaborate
on the disadvantages?

Guillermo



On Sep 27, 2:27 pm, pkenjora  wrote:
> Guillermo,
>
>   Having one application depend on another is OK.  If your are writing
> a generic reusable application that other can download and plug in
> then generic keys may be the way to go.  If however, you are simply
> having one of your own apps depend on another I strongly suggest not
> using generic foreign keys, they have drawbacks that could hinder you
> down the road.
>
> -Paul
>
> On Sep 27, 3:12 am, guillermooo 
> wrote:
>
>
>
> > Hi David,
>
> > I suppose that what you suggest would work too, but it would break the
> > reusability of the Todo application. I think what I need is rather a
> > GenericRelation/GenericForeignKey. Just found it in the docs.
>
> > Thanks,
>
> > Guillermo
>
> > On Sep 26, 10:21 pm, "djfis...@gmail.com" 
> > wrote:
>
> > > Guillermo,
>
> > > It is possible to have a model in one application have a foreign key
> > > to another application as of Django 1.0.
>
> > > From:http://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.mod...
> > > ---
> > > To refer to models defined in another application, you can explicitly
> > > specify a model with the full application label. For example, if the
> > > Manufacturer model above is defined in another application called
> > > production, you'd need to use:
>
> > > class Car(models.Model):
> > >     manufacturer = models.ForeignKey('production.Manufacturer')
>
> > > This sort of reference can be useful when resolving circular import
> > > dependencies between two applications.
> > > ---
>
> > > Hopefully this is what you need.
>
> > > -David
>
> > > On Sep 26, 12:07 pm, Guillermo 
> > > wrote:
>
> > > > Hi all,
>
> > > > I have one app with a Project model and another app with a TodoItem
> > > > model. How can I declare Project to be the foreign key of TodoItem?
> > > > Or, rather, how can I make TodoItem accept an arbitrary model as
> > > > foreign key?
>
> > > > Regards,
>
> > > > Guillermo
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Re: How to establish foreign key contraints across models living in different applications

[pkenjora]

Guillermo,

  Having one application depend on another is OK.  If your are writing
a generic reusable application that other can download and plug in
then generic keys may be the way to go.  If however, you are simply
having one of your own apps depend on another I strongly suggest not
using generic foreign keys, they have drawbacks that could hinder you
down the road.

-Paul

On Sep 27, 3:12 am, guillermooo 
wrote:
> Hi David,
>
> I suppose that what you suggest would work too, but it would break the
> reusability of the Todo application. I think what I need is rather a
> GenericRelation/GenericForeignKey. Just found it in the docs.
>
> Thanks,
>
> Guillermo
>
> On Sep 26, 10:21 pm, "djfis...@gmail.com" 
> wrote:
>
> > Guillermo,
>
> > It is possible to have a model in one application have a foreign key
> > to another application as of Django 1.0.
>
> > From:http://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.mod...
> > ---
> > To refer to models defined in another application, you can explicitly
> > specify a model with the full application label. For example, if the
> > Manufacturer model above is defined in another application called
> > production, you'd need to use:
>
> > class Car(models.Model):
> >     manufacturer = models.ForeignKey('production.Manufacturer')
>
> > This sort of reference can be useful when resolving circular import
> > dependencies between two applications.
> > ---
>
> > Hopefully this is what you need.
>
> > -David
>
> > On Sep 26, 12:07 pm, Guillermo 
> > wrote:
>
> > > Hi all,
>
> > > I have one app with a Project model and another app with a TodoItem
> > > model. How can I declare Project to be the foreign key of TodoItem?
> > > Or, rather, how can I make TodoItem accept an arbitrary model as
> > > foreign key?
>
> > > Regards,
>
> > > Guillermo
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Re: How to establish foreign key contraints across models living in different applications

[James Bennett]

On Sat, Sep 26, 2009 at 3:21 PM, djfis...@gmail.com
 wrote:
> It is possible to have a model in one application have a foreign key
> to another application as of Django 1.0.

It's always been possible to point relationships at models in other
applications. The bit you're linking to is a special alternate syntax
which lets you avoid circular import problems (where Model A in App A
and Model B in App B both refer to each other -- Python doesn't let
you import each one into the other's file), and that and *only* that
was new as of 1.0.


-- 
"Bureaucrat Conrad, you are technically correct -- the best kind of correct."

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Re: How to establish foreign key contraints across models living in different applications

[guillermooo]

Hi David,

I suppose that what you suggest would work too, but it would break the
reusability of the Todo application. I think what I need is rather a
GenericRelation/GenericForeignKey. Just found it in the docs.

Thanks,

Guillermo

On Sep 26, 10:21 pm, "djfis...@gmail.com" 
wrote:
> Guillermo,
>
> It is possible to have a model in one application have a foreign key
> to another application as of Django 1.0.
>
> From:http://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.mod...
> ---
> To refer to models defined in another application, you can explicitly
> specify a model with the full application label. For example, if the
> Manufacturer model above is defined in another application called
> production, you'd need to use:
>
> class Car(models.Model):
>     manufacturer = models.ForeignKey('production.Manufacturer')
>
> This sort of reference can be useful when resolving circular import
> dependencies between two applications.
> ---
>
> Hopefully this is what you need.
>
> -David
>
> On Sep 26, 12:07 pm, Guillermo 
> wrote:
>
>
>
> > Hi all,
>
> > I have one app with a Project model and another app with a TodoItem
> > model. How can I declare Project to be the foreign key of TodoItem?
> > Or, rather, how can I make TodoItem accept an arbitrary model as
> > foreign key?
>
> > Regards,
>
> > Guillermo
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Re: How to establish foreign key contraints across models living in different applications

[djfis...@gmail.com]

Guillermo,

It is possible to have a model in one application have a foreign key
to another application as of Django 1.0.

From:
http://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ForeignKey
---
To refer to models defined in another application, you can explicitly
specify a model with the full application label. For example, if the
Manufacturer model above is defined in another application called
production, you'd need to use:

class Car(models.Model):
manufacturer = models.ForeignKey('production.Manufacturer')

This sort of reference can be useful when resolving circular import
dependencies between two applications.
---

Hopefully this is what you need.

-David

On Sep 26, 12:07 pm, Guillermo 
wrote:
> Hi all,
>
> I have one app with a Project model and another app with a TodoItem
> model. How can I declare Project to be the foreign key of TodoItem?
> Or, rather, how can I make TodoItem accept an arbitrary model as
> foreign key?
>
> Regards,
>
> Guillermo
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How to establish foreign key contraints across models living in different applications

[Guillermo]

Hi all,

I have one app with a Project model and another app with a TodoItem
model. How can I declare Project to be the foreign key of TodoItem?
Or, rather, how can I make TodoItem accept an arbitrary model as
foreign key?

Regards,

Guillermo


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