Re: Help with #20378 - Reverse join fails when GenericRelation is defined on an abstract model

[Anssi Kääriäinen]

On 9 touko, 19:28, Gavin Wahl  wrote:
> I'd like to have a go at fixing this bug, but I'm not sure where to start.
> The problem seems to be that there's only a single instance of the field
> per model, so field.model is always the abstract class. I can't see a way
> to access the concrete class from the field instance. If the field only has
> a reference to the abstract model, which doesn't have a pk, how can it
> calculate join columns for the concrete model?

It can't. The field's model should be the concrete model, but due to
buggy field setup it isn't.

I took a look at altering the way virtual fields are copied from
abstract model to concrete model, but didn't find any easy solution
there. I didn't use too much time investigating this path, so taking
one more look might yield something. Start from
django.db.models.base:ModelBase.__new__() and virtual_fields copying.
Try to somehow end up in a situation where the field's model is the
concrete model.

More likely the solution is going to be found by altering how
GenericRelations are seen by the ORM. Before #19385 GenericRelation
masqueraded as m2m field (which it isn't), now in master it is a
ForeignObject subclass. The basic problem is that generic relation
represents reverse join, while ForeignObject usually represents direct
join. Doing changes in this area is likely going to be somewhat
complex...

 - Anssi

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Help with #20378 - Reverse join fails when GenericRelation is defined on an abstract model

[Gavin Wahl]

I'd like to have a go at fixing this bug, but I'm not sure where to start. 
The problem seems to be that there's only a single instance of the field 
per model, so field.model is always the abstract class. I can't see a way 
to access the concrete class from the field instance. If the field only has 
a reference to the abstract model, which doesn't have a pk, how can it 
calculate join columns for the concrete model?

https://code.djangoproject.com/ticket/20378

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