Re: Newbie question on re-defining an XML schema in my Django data model...

On Tue, Nov 29, 2011 at 3:44 PM, Marc Edwards <jmarcedwards@gmail.com> wrote:
> I need some help in resetting my thinking on creating my Django data
> model.
>
> I have previously created an XML schema definition for my data model,
> but am now trying to re-create this XML data model in a Django data
> model.
>
> In my XML schema, I had defined "collections" of XML complex types
> that essentially were lists that I could XQuery.  I used the ID &
> IDREF tags to cross-reference my elements between lists of elements.
> As XML is a tree structure, my schema defined a top down tree
> hierarchy.  Now I want to redefine this in a Django data model.
>
> In my primary scenario, I had a list of "singular" workflows that
> constituted a "parent" element "catalog" of workflows, i.e. a workflow
> catalog. Multiple "singular" workflows can be combined into an ORDERED
> collection of workflows called a "reference" or "composite" workflow.
> There are many composite/reference workflow instances with varied
> ordered combinations of singular workflows.
>
> In my XML hierarchy, I used the IDREF tags to reference the "singular"
> workflows from within the "reference" workflow element instance.
>
> In my Django object model, I can define a class for a "singular"
> workflow...easy enough, and I can query the Django database model to
> select the "singular" workflows.
>
> But, what is the correct method of now creating the "composite"
> workflow that will reference the singular workflows?
>
> My pseudo-code for the composite workflow class looks like this...
>
> class SingularWorkFlow(models.Model):
>   pass
>
> class CompositeWorkflow(models.Model):
>   OrderedListOfSingularWorkflows =
> list(models.ForeignKey(SingularWorkFlow)
>
> I don't think this will work, but would kindly ask for some guidance.
>
> Regards, Marc

"An ordered list of related models" sounds to me like "Many To Many
with a through model which has an ordering".

https://docs.djangoproject.com/en/1.3/topics/db/models/#intermediary-manytomany

You may want to specify a different manager for that relationship, so
that the querysets are always ordered correctly:

https://docs.djangoproject.com/en/1.3/topics/db/managers/#controlling-automatic-manager-types

On the other hand, you may not, ordering is expensive:

https://docs.djangoproject.com/en/1.3/ref/models/querysets/#order-by

Cheers

Tom

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