Fixed #26192 -- Fixed crash of ordering by constants on PostgreSQL.

[felixxm]

Ordering by constants is reasonable in some cases (e.g. in compound queries), but in the same time it can be problematic on PostgreSQL without specified output type (see ticket 26192).

IMO we should raise FieldError when output_field cannot be resolved.

@charettes What do you think?

[cansarigol]

I remembered @charettes pr about this.

[charettes]

@felixxm I'm not sure this is the right approach.

Somewhere between when the ticket was initially created (1.8 era) and now the annotate(constant=Value('foo')).order_by('constant') expression changed from being compiled to

SELECT ..., 'foo' AS constant ORDER BY 'foo'

to

SELECT ..., 'foo' AS constant ORDER BY constant

Which PostgreSQL doesn't have issues with.

The main difference now is that order_by reuses existing aliases which explains why your test_order_by_constant_value test passes. The original issue can still be reproduced by passing an expression directly to order_by though even if output_field is specified:

def test_order_by_constant_value(self):
    qs = Article.objects.order_by(
        constant=Value('1', output_field=IntegerField()),
    )
    list(qs)

The reason for this is that output_field resolution doesn't result in a cast (e.g. ::text) which is what seems to be tripping PostgreSQL here. Also the test_order_by_constant_value_without_output_field will to fail if #11359 is merged as @cansarigol pointed out.

Here's my take on how this could be resolved:

1. Make order_by simply ignore Value expressions. They are constants anyway so there's no point in ordering by them.
2. Require that Value expressions passed to order_by be resolvable like you did here and wrap them in Cast(val, val.output_field).

[felixxm]

@charettes Thanks for your analysis 🙇‍♂️ I added Cast() expressions and extra tests.

I agree that original issue can be reproduced by passing an expression directly to order_by but only when output_field is not an IntegerField, because integers are interpreted as column numbers (what is misleading).

I also know test_order_by_constant_value_without_output_field will not raise exceptions after #11359 but that's OK (IMO) we can move these use cases into test_order_by_constant_value.

[charettes]

Looks great, thanks for the adjustments.

[felixxm]

@charettes Thanks!