I wish to filter some database objects with a concatenated string.

The standard SQL query could be:

SELECT concat(firstName, ' ', name) FROM person WHERE CONCAT(firstName, ' ', name) LIKE "a%";

Within the model, I've produced a supervisor known as PersonObjects:

class PersonObjects(Manager):
    attrs = { 
        'fullName': "CONCAT(firstName, ' ', name)"
    }   

    def get_query_set(self):
        return super(PersonObjects, self).get_query_set().extra(
            select=self.attrs)

I additionally set up this during my model:

objects = managers.PersonObjects()

Now being able to access fullName works best for single objects:

>>> p = models.Person.objects.get(pk=4)
>>> p.fullName
u'Fred Borminski'

But it doesn't operate in a filter:

>>> p = models.Person.objects.filter(fullName__startswith='Alexei')
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/django/db/models/manager.py", line 141, in filter
    return self.get_query_set().filter(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 550, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 568, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1128, in add_q
    can_reuse=used_aliases)
  File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1026, in add_filter
    negate=negate, process_extras=process_extras)
  File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1191, in setup_joins
    "Choices are: %s" % (name, ", ".join(names)))
FieldError: Cannot resolve keyword 'fullName' into field. Choices are: firstName, gender, name, (...)

Is a bug or perhaps a feature? How do i fix this?

Thanks.

It isn't a bug. filter() only inspects model definitions, therefore it does not recognize fullName like a declared area (since it is not - this is an extra argument inside a query).

You can include the fullName to WHERE using extra():

Person.objects.extra(where=["fullName LIKE %s"], params=["Alexei%"])