Re: ForeignKey Reverse Relation Fail

Hi
Have you tried using OneToMany relationship ?

On 27 Jul 2022 12:18, "Malik Rumi" <malik.a.rumi@gmail.com> wrote:

I have a model with a recursive foreign key to 'self'. This is intended to model a parent child
relation among instances. The forward relation, on a field called 'childof', works as expected.
The reverse relation, using the related_name 'parent', comes up as a RelatedManager,
again as expected. However, parent.count(), parent.all(), etc., always give me the "Manager is
not accessible on instances" error. Many of these parent instances will themselves also be a
childof some other instance, and apparently, that's my problem. I don't know how to make Django
recognize the dual nature of some instances. Is there a way to hack the RelatedManager to fix this?

I am not getting any accessor errors from manage.py check.

I posted this to Django Forum, but the respondent was not able to duplicate my issue - i.e.,
he said it worked as expected for him. :-(

I saw a suggestion to use an explicit junction table on Stack Overflow, but making a round trip - or two - to an external table seems like an
awful lot of overhead for this situation.

If instead of a junction table, if I made an explicit parentto field on the model, would that work?
Presumably the related name on the childof field would still fail like it does now,
but I would instead have the explicit parent field to work with. I still would not have the
automatic reverse relation, and I would have to come up with a script to fill in the parentto
field, but that might solve my problem:
# pseudocode
family = c.itertools.groupby(instance.childof)
family = c.pandas.groupby(instance.childof)
for f in family:
pop = c.objects.get(instance.childof)
OR
pop = instance.childof
c.objects.update(pop.parentto=f) # where parentto is a Postgresql ArrayField
OR # https://docs.djangoproject.com/en/4.0/ref/models/relations/#django.db.models
.fields.related.RelatedManager.add:
>>> b = Blog.objects.get(id=1)
>>> e = Entry.objects.get(id=234)
>>> b.entry_set.add(e) # Associates Entry e with Blog b
SEE ALSO: https://docs.djangoproject.com/en/4.0/topics/db/examples/many_to_one/

If I did this two field hack, then I don't really need either one to be ForeignKeys any more,
do I? They could be a CharField and an ArrayField, couldn't they? That breaks the extended
lookup chain - but I don't have that now, anyway - or at least, I only have it in one direction.

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