Cast function accepts field class according to docs but expects field instance

[Valentin Ignatyev]

Example from docs about django.db.models.functions.Cast will fail with TypeError: db_type() missing 1 required positional argument: 'connection' because in as_sql method when adding db_type to extra_context we execute db_type method of field class and not field instance:

 if 'db_type' not in extra_context:
    extra_context['db_type'] = self._output_field.db_type(connection)

Here self._output_field is a class instead of instance. So we either must pass an instance of field like

value = Value.objects.annotate(as_float=Cast('integer', FloatField())).get()

and point to it in docs or create instance inside as_sql like this:

extra_context['db_type'] = self._output_field().db_type(connection)

[Simon Charette]

Here's a ​PR with the documentation adjustments.

[Tim Graham <timograham@…>]

In 767ba009:

Fixed #27209 -- Fixed typo in docs/ref/models/database-functions.txt.

Thanks Valentin Ignatyev for the report.

[Tim Graham <timograham@…>]

In 23cf7e5:

[1.10.x] Fixed #27209 -- Fixed typo in docs/ref/models/database-functions.txt.

Thanks Valentin Ignatyev for the report.

Backport of 767ba009764c6640b20ea6a6dc6ee2559cc87800 from master