DatabaseError on using empty Page .object_list in __in clause in a query

[chkwok@…]

I've noticed that after upgrading to Django 1.4, in queries really don't like empty sets. Simple queries still work, like User.objects.filter(groupsin=[]), but most failures I've seen are with Paginators. I think this is the minimum set to cause a DatabaseError, create any app, add a models.py with:

from django.db import models

class Author(models.Model):
    pass

class Book(models.Model):
    author = models.ForeignKey(Author)

def crash():
    from django.core.paginator import Paginator
    
    pages = Paginator(Author.objects.all(), 25)
    page = pages.page(1)
    
    books = Book.objects.filter(author__in=page.object_list)
    print books

calling crash() will cause this stack trace:

C:\Workspace\someproject\src\someproject\test.py in <module>()
      6
      7 books = Book.objects.filter(author__in=page.object_list)
----> 8 print books
      9

C:\Dev\Python27\lib\site-packages\django\db\models\query.pyc in __repr__(self)
     70
     71     def __repr__(self):
---> 72         data = list(self[:REPR_OUTPUT_SIZE + 1])
     73         if len(data) > REPR_OUTPUT_SIZE:
     74             data[-1] = "...(remaining elements truncated)..."

C:\Dev\Python27\lib\site-packages\django\db\models\query.pyc in __len__(self)
     85                 self._result_cache = list(self.iterator())
     86         elif self._iter:
---> 87             self._result_cache.extend(self._iter)
     88         if self._prefetch_related_lookups and not self._prefetch_done:
     89             self._prefetch_related_objects()

C:\Dev\Python27\lib\site-packages\django\db\models\query.pyc in iterator(self)
    289             klass_info = get_klass_info(model, max_depth=max_depth,
    290                                         requested=requested, only_load=only_load)
--> 291         for row in compiler.results_iter():
    292             if fill_cache:
    293                 obj, _ = get_cached_row(row, index_start, db, klass_info,

C:\Dev\Python27\lib\site-packages\django\db\models\sql\compiler.pyc in results_iter(self)
    761         if self.query.select_for_update and transaction.is_managed(self.using):
    762             transaction.set_dirty(self.using)
--> 763         for rows in self.execute_sql(MULTI):
    764             for row in rows:
    765                 if resolve_columns:

C:\Dev\Python27\lib\site-packages\django\db\models\sql\compiler.pyc in execute_sql(self, result_type)
    816
    817         cursor = self.connection.cursor()
--> 818         cursor.execute(sql, params)
    819
    820         if not result_type:

C:\Dev\Python27\lib\site-packages\django\db\backends\util.pyc in execute(self, sql, params)
     38         start = time()
     39         try:
---> 40             return self.cursor.execute(sql, params)
     41         finally:
     42             stop = time()

C:\Dev\Python27\lib\site-packages\django\db\backends\postgresql_psycopg2\base.pyc in execute(self, query, args)
     50     def execute(self, query, args=None):
     51         try:
---> 52             return self.cursor.execute(query, args)
     53         except Database.IntegrityError, e:
     54             raise utils.IntegrityError, utils.IntegrityError(*tuple(e)), sys.exc_info()[2]

DatabaseError: syntax error at or near ")"
LINE 1: ...ugtest_book" WHERE "bugtest_book"."author_id" IN () LIMIT 21

The SQL statement created is:

SELECT "bugtest_book"."id", "bugtest_book"."author_id" FROM "bugtest_book" WHERE "bugtest_book"."author_id" IN () LIMIT 21