I will not say that it is pretty, but, I looked into doing this a
while ago for an app I am developing. I wanted to create a system
that could inventory and track many different types of assets, and
allow for users to essentially create additional asset types on the
fly. Essentially, I decided upon using meta-tables to provide the
flexibility, it passed the smell-test with a couple of database
architect friends of mine (not my specialty), but was essentially too
complex to provide enough return for my particular situation. What I
came up with is something like this (code is abbreviated, and this is
off the top of my head):
class attributetype(model):
data_type_choices=(bool, text, char, int)
name = charfield()
number_allowed = intfield()
data_type = charfield(choices=data_type_choices)
class attribute(model):
type = foreignkey(attributetype)
booldata = booleanfield()
textdata = charfield()
intdata = integerfield()
def __str__(self):
if type.data_type == text:
return('%s' % textdata)
if type.data_type == char:
etc....
class AssetType(Model):
name = charfield()
attribute_types = manytomany(attributetype)
class Asset(Model):
name = models.charfield()
type = models.foreignkey(assettype)
attributes = manytomany(attributes)
Building a form for adding or updating an Asset was obviously not
trivial. You would have to write a function that would look at the
asset type, and create formfields for the asset based on what
attributetypes and the number allowed by the attribute defined. This
did mean though, that if I had two assets that shared an attribute but
had conflicting numbers allowed, I needed to create two separate
attribute types. Using (something like) this I could create any kind
of asset I wanted to and create the necessary attributes required to
store pertinent information. Like I said, the complexity outweighed
what I believed to be the return in my particular case. Hope this
helps.
-rfd
On Aug 24, 11:29 am, "Nathaniel Martin" <[EMAIL PROTECTED]>
wrote:
> Hmmm, that's an interesting method. I was hoping for some way to add
> attributes though. Does anyone else have any ideas?
> -Nate
>
> On 8/23/07, Thomas Guettler <[EMAIL PROTECTED]> wrote:
>
>
>
> > Am Freitag, 24. August 2007 00:50 schrieb Nathaniel Martin:
> > > I'm hoping that some of the django experts on this list can help me with
> > a
> > > problem I'm working on designing the architecture of a site I'm working
> > on.
> > > I want to have many objects that each belong to a category. Each
> > category
> > > has a bunch of attributes. Each object would set values for each of
> > those
> > > attributes.
>
> > ...
> > [cut]
> > > B) Have an 'Objects' table, a 'Categories' table, and multiple
> > 'Attributes'
> > > tables, one for each datatype I think will be used. The 'Objects' table
> > has
> > > a column tying it to a certain category. The 'Categories' table has a
> > > column for each attribute table, and lists which attributes are used for
> > > which table. Each 'Attribute' table stores all the attributes of that
> > > datatype, with a column pointing to which category and object it's for.
>
> > > Downsides: Really complicated. I can see this getting very messy very
> > > quickly. Lots of tables.
>
> > I have started an application that uses this approach. It is far from
> > being
> > finished. I do something like inheritance at database level:
>
> > One Category can have an other Category as parent. A subcategory has
> > all the attributes of its parent and grandparents.
>
> > The user can:
> > - change the hierarchie of categories
> > - add categories
> > - add existing attributes to categories
>
> > Up to now he can't:
> > - add new attributes. (
> > This would need a python file and something like syncdb.
> > That should be done by a developer)
>
> > Thomas
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