On 8/08/2011 8:42pm, Michal Petrucha wrote:
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...
Thanks Michal - doing so will simplify things. I think I can make it
more hierarchical.
Cheers
Mike
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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