On Mon, Aug 08, 2011 at 06:28:59PM +1000, Mike Dewhirst wrote:
> Can anyone tell me if to_field is valid in this context and what is
> the distinction between through and db_table? ...
>
> class Sections(models.Model):
> """ intermediary table for Document recursive m2m """
> document = models.ForeignKey(Document, to_field='ancestor')
> section = models.ForeignKey(Document, to_field='ancestor')
> comment = models.TextField()
>
> class Document(models.Model):
> ancestor = models.IntegerField()
> sections = models.ManyToManyField('self',
> symmetrical=False,
> through=Sections,
> db_table=Sections,
> related_name='Comprise')``db_table`` is ignored altogether if you supply a ``through`` model. Otherwise it is expected to be a string containing the table name. > ... where 'ancestor' is a surrogate key which survives in descendant > documents - such being both newer revisions of the same document and > other documents more or less based on the ancestral document. > > I also wonder how I might have a m2m recursive relationship between > documents without specifying an intermediary table - iow if the > comment column was not needed - is it OK to specify > to_field='ancestor' in the Document model? In general, this won't work. ManyToManyField doesn't really support pointing to any other field than the primary key. That's why it doesn't have the ``to_field`` option at all. Note that while the code above might work under some circumstances, it will break as soon as you start working with ModelForms -- the form field will feed Document.ancestor with values from Document.pk which might reault in a complete mess. And that is just one example of what won't work... All in all, I recommend redesigning the solution to avoid such usage of ManyToManyField. I don't really understand what it is you're trying to achieve but if you are somehow grouping the documents using the ancestor field, you might try to separate the groups into a new model or something... But again, I don't quite understand what you're doing egen after re-reading your post from about a week ago... Michal
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