Amateur question on class definitions...

[Ian Kelly]

On Tue, Nov 29, 2011 at 8:21 AM, J. Marc Edwards <jmarcedwards at gmail.com> wrote:
> So I am defining my Python classes for a Django data model.  I have a the
> following class hierarchy and would value some expert input on how to best
> implement this.
>
> I have many instances of a particular class which I call a "workflow", e.g.
> wf_1, wf_2, wf_3, etc.
> These class instances of workflows can be combined into "composite
> workflows", e.g. {cwf_1 is comprised of wf1 & wf_3}, {cwf_2 is wf_2 and
> wf_3}, or {cwf_3 is just wf_2}, etc.
> The workflows that constitute the composite workflow is positionally
> dependent, i.e. if cwf_1 is comprised of wf_1 and wf_3, then wf_1 comes 1st
> AND wf_3 is 2nd.
> As I have many workflow instances that accumulate in my persistent database
> (Django data model), then the composite workflows become an ordered
> collection of workflow instances.
>
> My first thought here is that the composite workflow class should have a
> field that is a simple list of workflows, i.e. list(workflows), but I am
> somewhat unsure of how to code this correctly.
>
> Here is a sample pseudo-code...
>
> class a_workflow(models.Model):
> ...
>
> class composite_workflow(models.Model):
>     ...a list of "a_workflows", i.e. a composite workflow, should I use this
> syntax?
>     set_of_workflows = list(a_workflow)
>
> Is this how a pro would do this?

That would work in general, but since these are Django models I assume
that this relationship will need to be stored in the database.  You'll
need to use a ManyToManyField instead.  I would do something like
this:

class Workflow(models.Model):
    pass

class CompositeWorkflow(models.Model):
    workflow_set = models.ManyToManyField('Workflow',
through='CompositeWorkflowOrder')

class CompositeWorkflowOrder(models.Model):
    workflow = models.ForeignKey('Workflow')
    composite_workflow = models.ForeignKey('CompositeWorkflow')
    sequence_num = models.IntegerField()

The CompositeWorkflowOrder model is then responsible for tracking
which workflows are contained by which composite workflows, and in
what order they appear (using sequence_num).  See the Django docs on
ManyToManyFields for more info.

Cheers,
Ian