extra fields of Model.objects.extra() can't be filtered

[fetzig]

sepp = SomeModel.objects.extra(select={'hiho': """(6371 * acos(cos( radians(%f) ) 
* cos( radians( geo_lat ) ) * cos( radians( geo_long ) - radians(%f) ) 
+ sin( radians(%f) ) * sin( radians( geo_lat ) ) ))""" % (latitude, longitude, latitude)})
sepp.filter(hiho__lt=30)

throws an exception:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/django/db/models/query.py", line 550, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/django/db/models/query.py", line 568, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/django/db/models/sql/query.py", line 1131, in add_q
    can_reuse=used_aliases)
  File "/django/db/models/sql/query.py", line 1026, in add_filter
    negate=negate, process_extras=process_extras)
  File "/django/db/models/sql/query.py", line 1194, in setup_joins
    "Choices are: %s" % (name, ", ".join(names)))
FieldError: Cannot resolve keyword 'hiho' into field. Choices are: ...all the fields of the model

[Alex Gaynor]

This is intentional, you need to use the where option of extra() to filter by extra select items.

[ris]

Sorry I know it's impolite to reopen tickets like this.

I too would find this a very useful feature.

I realize _similar_ functionality is possible using the where= clause, but allowing the selected parameter to be filtered against would mean you could re-use the same calculated parameter (multiple times) from within complex Q() statements.

e.g. - currently in my app I am dynamically building a really very complex query using Q statements. Having to fall back to the QuerySet to perform the extra() from within this code would be quite painful and screw up my encapsulation.

[Russell Keith-Magee]

As Alex notes, this behavior is intentional. extra() is designed for inserting specific SQL into a query, not as a generic extension mechanism for the ORM.

It sounds like what you actually want is the ability to include model annotations that aren't aggregates.