Re: Model Inheritance and ModelForms

Tom,

Thanks for the quick reply! However, when I tried that I got an error
saying that inventoryitem "is an invalid keyword argument for this
function". I fixed this by manually specifying the OneToOneField
relationship with the parent class as inventoryItem and setting
parent_link=True. With that in place, I get errors about InventoryItem
fields not being allowed to be null within InventoryBook. This doesn't
make sense to me since InventoryBook should only have a link to the
parent and its own local book-specific fields - not the fields
previously declared in the parent InventoryItem. I have not declared
any models to be abstract (and in fact have not declared Meta: at
all).

If I need to manually specify all of the InventoryItem fields again in
the constructor of the InventoryBook, I can. But doing so seems to
defeat the purpose of using inheritance to begin with. Is there
something I'm still overlooking?

Thanks again,

-Alex

On Oct 28, 5:53 am, Tom Evans <tevans...@googlemail.com> wrote:
> On Fri, Oct 28, 2011 at 7:50 AM, Alex <alexc...@gmail.com> wrote:
> > I've been scouring Google and the Django documentation and I can't
> > figure out how to do this. I'm working on an inventory management
> > system for my shop. My inventory models.py similar to the following:
>
> >    class ItemType( models.Model ):
> >        def __unicode__( self ):
> >            return "blah blah blah"
>
> >        itemType = models.CharField( max_length = 32 )
> >        isBook = models.BooleanField()
>
> >    class InventoryItem( models.Model ):
> >        def __unicode__( self ):
> >            return "blah"
>
> >        itemType = models.ForeignKey( ItemType )
> >        description = models.CharField( max_length = 256 )
>
> >    class InventoryBook( InventoryItem ):
> >        def __unicode__( self ):
> >            return "blah blah"
>
> >        title = models.CharField( max_length = 64 )
>
> > In my web app, I create a ModelForm based on InventoryItem and present
> > that to the user. Upon submission, the POST data is used to create an
> > instance of InventoryItem. I then check to see if
> > inventoryItem.itemType.isBook() is True - if True, I want to cast
> > inventoryItem to an InventoryBook type so I can set the extra fields
> > and call save() so that it creates records in both tables in the MySQL
> > database.
>
> > I started trying to add a method to InventoryItem that would return an
> > InventoryBook instance after being given the title string, but that
> > doesn't work because Python doesn't have prototyping/forward
> > declarations.
>
> > Am I approaching this completely wrong, or am I just overlooking
> > something simple? Any advice or links to relevant documentation would
> > be *much* appreciated. Thanks!
>
> > -Alex
>
> If you have an InventoryItem which should be an InventoryBook, you
> need to create the InventoryBook, not cast to it.
>
> item = …
> if item.itemType.isBook():
>   book = InventoryBook.objects.create(inventoryitem=item, title=…)
>
> Inheritance like this in django isn't really like OOO inheritance,
> InventoryBook just magically gains a OneToOneField back to
> InventoryItem called inventoryitem.
>
> Cheers
>
> Tom

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