Re: Multi-table inheritance - knowing the child object from the parent

[Jori]

http://docs.djangoproject.com/en/1.2/topics/db/models/#id7

You can access sub-classes with place.restaurant and place.gardenstore
if the model has them.

I did something similar the other day but added an extra field to
parent class to indicate which type of a child model it had (1 for
Restaurant etc). I don't know if this is a proper way to handle the
situation but it works for me.

-Jori

On May 18, 7:11 pm, Lee Hinde  wrote:
> On Tue, May 18, 2010 at 4:27 AM, Jani Tiainen  wrote:
> > > Given the following:
>
> > > class Place(models.Model):
> > >     name = models.CharField(max_length=50)
> > >     address = models.CharField(max_length=80)
> > >     zip_code = models.CharField(max_length=15)
>
> > > class Restaurant(Place):
> > >     serves_hot_dogs = models.BooleanField()
> > >     serves_pizza = models.BooleanField()
>
> > > class GardenStore(Place):
> > >     sells_lady_bugs = models.BooleanField()
>
> > > and given a query on zip_code in Places, how would I know whether a
> > > Place is a Restaurant or a Garden Store?
>
> > You don't OOTB.
>
> > I asked same thing quite a while ago - you should be able to find my
> > thoughts
> > somewhere in this group archives.
>
> > Rationale behind this is that you're trying to achieve something pretty
> > much
> > impossible - inheritance in relational world.
>
> > Thing is that you can have you can really have both: Restaurant and
> > GardenStore instance to point to same place - so is that correct case or
> > not?
>
> > Note that "inheritance" is made so that you have relation from Restaurant
> > and
> > GardenStore tables to Place. There is no simple mechanism to prevent your
> > data
> > to have references from both "children" to point to "parent".
>
> > You can have property/method on your model that tries to determine which
> > one
> > is correct.
>
> > Or you can re-factor your models (which is usually less painful) so that
> > your
> > place, provided that it is unique (which actually in this case might not
> > be).
> > Add simple discriminator field to tell what type of child you must also
> > fetch
> > to make data complete.
>
>  Thanks Jani; I'd seen some of the older threads, with similar work arounds,
> but those discussions pre-dated recent advances. I'll look again for the
> thread you mention.
>
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Re: Multi-table inheritance - knowing the child object from the parent

[Lee Hinde]

On Tue, May 18, 2010 at 4:27 AM, Jani Tiainen  wrote:

> > Given the following:
> >
> > class Place(models.Model):
> > name = models.CharField(max_length=50)
> > address = models.CharField(max_length=80)
> > zip_code = models.CharField(max_length=15)
> >
> >
> > class Restaurant(Place):
> > serves_hot_dogs = models.BooleanField()
> > serves_pizza = models.BooleanField()
> >
> > class GardenStore(Place):
> > sells_lady_bugs = models.BooleanField()
> >
> >
> > and given a query on zip_code in Places, how would I know whether a
> > Place is a Restaurant or a Garden Store?
>
> You don't OOTB.
>
> I asked same thing quite a while ago - you should be able to find my
> thoughts
> somewhere in this group archives.
>
> Rationale behind this is that you're trying to achieve something pretty
> much
> impossible - inheritance in relational world.
>
> Thing is that you can have you can really have both: Restaurant and
> GardenStore instance to point to same place - so is that correct case or
> not?
>
> Note that "inheritance" is made so that you have relation from Restaurant
> and
> GardenStore tables to Place. There is no simple mechanism to prevent your
> data
> to have references from both "children" to point to "parent".
>
> You can have property/method on your model that tries to determine which
> one
> is correct.
>
> Or you can re-factor your models (which is usually less painful) so that
> your
> place, provided that it is unique (which actually in this case might not
> be).
> Add simple discriminator field to tell what type of child you must also
> fetch
> to make data complete.
>
>
>
 Thanks Jani; I'd seen some of the older threads, with similar work arounds,
but those discussions pre-dated recent advances. I'll look again for the
thread you mention.

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Re: Multi-table inheritance - knowing the child object from the parent

[Jani Tiainen]

> Given the following:
> 
> class Place(models.Model):
> name = models.CharField(max_length=50)
> address = models.CharField(max_length=80)
> zip_code = models.CharField(max_length=15)
> 
> 
> class Restaurant(Place):
> serves_hot_dogs = models.BooleanField()
> serves_pizza = models.BooleanField()
> 
> class GardenStore(Place):
> sells_lady_bugs = models.BooleanField()
> 
> 
> and given a query on zip_code in Places, how would I know whether a
> Place is a Restaurant or a Garden Store?

You don't OOTB.

I asked same thing quite a while ago - you should be able to find my thoughts 
somewhere in this group archives.

Rationale behind this is that you're trying to achieve something pretty much 
impossible - inheritance in relational world. 

Thing is that you can have you can really have both: Restaurant and 
GardenStore instance to point to same place - so is that correct case or not?

Note that "inheritance" is made so that you have relation from Restaurant and 
GardenStore tables to Place. There is no simple mechanism to prevent your data 
to have references from both "children" to point to "parent".

You can have property/method on your model that tries to determine which one 
is correct.

Or you can re-factor your models (which is usually less painful) so that your 
place, provided that it is unique (which actually in this case might not be). 
Add simple discriminator field to tell what type of child you must also fetch 
to make data complete.

-- 

Jani Tiainen

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Multi-table inheritance - knowing the child object from the parent

[Lee Hinde]

Given the following:

class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
zip_code = models.CharField(max_length=15)


class Restaurant(Place):
serves_hot_dogs = models.BooleanField()
serves_pizza = models.BooleanField()

class GardenStore(Place):
sells_lady_bugs = models.BooleanField()


and given a query on zip_code in Places, how would I know whether a
Place is a Restaurant or a Garden Store?

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