id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
34552	Delaying get_from_clause call as much as possible.	Matthew Schinckel	Matthew Schinckel	"I've got a couple of projects that supplement ORM functionality. One in particular is [django-shared-property](https://github.com/schinckel/django-shared-property), which allows a field to be defined on the model, that will automatically be added as an annotation, and as such can be filtered/aggregated.

However, it's also ""live"" in the sense that changes made after loading from the database to dependencies of the property will be reflected.

For example:


{{{
class Ordering(models.Model):
    a = models.IntegerField()
    b = models.IntegerField()

class MyModel(models.Model):
    ord = models.ForeignKey(Ordering)
    
    @shared_property(models.F('ord__a') / models.F('ord__b'))
    def ordering(self):
        return self.ord.a / self.ord.b
}}}

In this case, you can filter/order by `ordering`, but also if you changed `ord.a`, then `ordering` would update accordingly, without having to save the object and refetch.

Anyway, long story short, this code works amazingly well without any changes to the Django code, except for one edge case: when a shared_property refers to another model (which on it's own works), but you then filter on this shared_property (which again, works) and then do a `.count()` or `.exists()`.


{{{
MyModel.objects.filter(ordering__gte=10).count()
}}}


It turns out that because of the way the ORM compiler works, `get_from_clause` is called before any filters are applied. Which normally works okay, but in this case results in the table not being available.

A solution I came up with was to delay the evaluation of `get_from_clause` until after the where/having clauses have been processed, allowing them to flag tables that need to be referenced.

As such, I'd like to propose the change in https://github.com/django/django/pull/14683, which passes django tests (and also fixes the problem for me).
"	New feature	closed	Database layer (models, ORM)	4.2	Normal	needsinfo		Simon Charette Florian Apolloner	Unreviewed	1	0	0	0	0	0