Added support for modifying the effect of ``DISTINCT`` clauses so they

only consider some fields (PostgreSQL only).

For this, the ``distinct()`` QuerySet method now accepts an optional
list of model fields names and generates ``DISTINCT ON`` clauses on
these cases. Thanks Jeffrey Gelens and Anssi Kääriäinen for their work.

Fixes #6422.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17244 bcc190cf-cafb-0310-a4f2-bffc1f526a37

AUTHORS

@@ -203,6 +203,7 @@ answer newbie questions, and generally made Django that much better:
     Marc Garcia <marc.garcia@accopensys.com>
     Andy Gayton <andy-django@thecablelounge.com>
     geber@datacollect.com
+    Jeffrey Gelens <jeffrey@gelens.org>
     Baishampayan Ghose
     Joshua Ginsberg <jag@flowtheory.net>
     Dimitris Glezos <dimitris@glezos.com>
@@ -269,6 +270,7 @@ answer newbie questions, and generally made Django that much better:
     jpellerin@gmail.com
     junzhang.jn@gmail.com
     Xia Kai <http://blog.xiaket.org/>
+    Anssi Kääriäinen
     Antti Kaihola <http://djangopeople.net/akaihola/>
     Peter van Kampen
     Bahadır Kandemir <bahadir@pardus.org.tr>

django/db/backends/__init__.py

@@ -406,6 +406,9 @@ class BaseDatabaseFeatures(object):
     supports_stddev = None
     can_introspect_foreign_keys = None
 
+    # Support for the DISTINCT ON clause
+    can_distinct_on_fields = False
+
     def __init__(self, connection):
         self.connection = connection
 
@@ -559,6 +562,17 @@ class BaseDatabaseOperations(object):
         """
         raise NotImplementedError('Full-text search is not implemented for this database backend')
 
+    def distinct_sql(self, fields):
+        """
+        Returns an SQL DISTINCT clause which removes duplicate rows from the
+        result set. If any fields are given, only the given fields are being
+        checked for duplicates.
+        """
+        if fields:
+            raise NotImplementedError('DISTINCT ON fields is not supported by this database backend')
+        else:
+            return 'DISTINCT'
+
     def last_executed_query(self, cursor, sql, params):
         """
         Returns a string of the query last executed by the given cursor, with

django/db/backends/postgresql_psycopg2/base.py

@@ -82,6 +82,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_select_for_update_nowait = True
     has_bulk_insert = True
     supports_tablespaces = True
+    can_distinct_on_fields = True
 
 class DatabaseWrapper(BaseDatabaseWrapper):
     vendor = 'postgresql'

django/db/backends/postgresql_psycopg2/operations.py

@@ -179,6 +179,12 @@ class DatabaseOperations(BaseDatabaseOperations):
 
         return 63
 
+    def distinct_sql(self, fields):
+        if fields:
+            return 'DISTINCT ON (%s)' % ', '.join(fields)
+        else:
+            return 'DISTINCT'
+
     def last_executed_query(self, cursor, sql, params):
         # http://initd.org/psycopg/docs/cursor.html#cursor.query
         # The query attribute is a Psycopg extension to the DB API 2.0.

django/db/models/query.py

@@ -323,6 +323,8 @@ class QuerySet(object):
         If args is present the expression is passed as a kwarg using
         the Aggregate object's default alias.
         """
+        if self.query.distinct_fields:
+            raise NotImplementedError("aggregate() + distinct(fields) not implemented.")
         for arg in args:
             kwargs[arg.default_alias] = arg
 
@@ -751,12 +753,14 @@ class QuerySet(object):
         obj.query.add_ordering(*field_names)
         return obj
 
-    def distinct(self, true_or_false=True):
+    def distinct(self, *field_names):
         """
         Returns a new QuerySet instance that will select only distinct results.
         """
+        assert self.query.can_filter(), \
+                "Cannot create distinct fields once a slice has been taken."
         obj = self._clone()
-        obj.query.distinct = true_or_false
+        obj.query.add_distinct_fields(*field_names)
         return obj
 
     def extra(self, select=None, where=None, params=None, tables=None,
@@ -1179,7 +1183,7 @@ class EmptyQuerySet(QuerySet):
         """
         return self
 
-    def distinct(self, true_or_false=True):
+    def distinct(self, fields=None):
         """
         Always returns EmptyQuerySet.
         """

django/db/models/sql/compiler.py

@@ -23,6 +23,8 @@ class SQLCompiler(object):
         Does any necessary class setup immediately prior to producing SQL. This
         is for things that can't necessarily be done in __init__ because we
         might not have all the pieces in place at that time.
+        # TODO: after the query has been executed, the altered state should be
+        # cleaned. We are not using a clone() of the query here.
         """
         if not self.query.tables:
             self.query.join((None, self.query.model._meta.db_table, None, None))
@@ -60,11 +62,19 @@ class SQLCompiler(object):
             return '', ()
 
         self.pre_sql_setup()
+        # After executing the query, we must get rid of any joins the query
+        # setup created. So, take note of alias counts before the query ran.
+        # However we do not want to get rid of stuff done in pre_sql_setup(),
+        # as the pre_sql_setup will modify query state in a way that forbids
+        # another run of it.
+        self.refcounts_before = self.query.alias_refcount.copy()
         out_cols = self.get_columns(with_col_aliases)
         ordering, ordering_group_by = self.get_ordering()
 
-        # This must come after 'select' and 'ordering' -- see docstring of
+        distinct_fields = self.get_distinct()
-        # get_from_clause() for details.
+
+        # This must come after 'select', 'ordering' and 'distinct' -- see
+        # docstring of get_from_clause() for details.
         from_, f_params = self.get_from_clause()
 
         qn = self.quote_name_unless_alias
@@ -76,8 +86,10 @@ class SQLCompiler(object):
             params.extend(val[1])
 
         result = ['SELECT']
+
         if self.query.distinct:
-            result.append('DISTINCT')
+            result.append(self.connection.ops.distinct_sql(distinct_fields))
+
         result.append(', '.join(out_cols + self.query.ordering_aliases))
 
         result.append('FROM')
@@ -90,6 +102,9 @@ class SQLCompiler(object):
 
         grouping, gb_params = self.get_grouping()
         if grouping:
+            if distinct_fields:
+                raise NotImplementedError(
+                    "annotate() + distinct(fields) not implemented.")
             if ordering:
                 # If the backend can't group by PK (i.e., any database
                 # other than MySQL), then any fields mentioned in the
@@ -129,6 +144,9 @@ class SQLCompiler(object):
                 raise DatabaseError('NOWAIT is not supported on this database backend.')
             result.append(self.connection.ops.for_update_sql(nowait=nowait))
 
+        # Finally do cleanup - get rid of the joins we created above.
+        self.query.reset_refcounts(self.refcounts_before)
+
         return ' '.join(result), tuple(params)
 
     def as_nested_sql(self):
@@ -292,6 +310,26 @@ class SQLCompiler(object):
                     col_aliases.add(field.column)
         return result, aliases
 
+    def get_distinct(self):
+        """
+        Returns a quoted list of fields to use in DISTINCT ON part of the query.
+
+        Note that this method can alter the tables in the query, and thus it
+        must be called before get_from_clause().
+        """
+        qn = self.quote_name_unless_alias
+        qn2 = self.connection.ops.quote_name
+        result = []
+        opts = self.query.model._meta
+
+        for name in self.query.distinct_fields:
+            parts = name.split(LOOKUP_SEP)
+            field, col, alias, _, _ = self._setup_joins(parts, opts, None)
+            col, alias = self._final_join_removal(col, alias)
+            result.append("%s.%s" % (qn(alias), qn2(col)))
+        return result
+
+
     def get_ordering(self):
         """
         Returns a tuple containing a list representing the SQL elements in the
@@ -384,21 +422,7 @@ class SQLCompiler(object):
         """
         name, order = get_order_dir(name, default_order)
         pieces = name.split(LOOKUP_SEP)
-        if not alias:
+        field, col, alias, joins, opts = self._setup_joins(pieces, opts, alias)
-            alias = self.query.get_initial_alias()
-        field, target, opts, joins, last, extra = self.query.setup_joins(pieces,
-                opts, alias, False)
-        alias = joins[-1]
-        col = target.column
-        if not field.rel:
-            # To avoid inadvertent trimming of a necessary alias, use the
-            # refcount to show that we are referencing a non-relation field on
-            # the model.
-            self.query.ref_alias(alias)
-
-        # Must use left outer joins for nullable fields and their relations.
-        self.query.promote_alias_chain(joins,
-            self.query.alias_map[joins[0]][JOIN_TYPE] == self.query.LOUTER)
 
         # If we get to this point and the field is a relation to another model,
         # append the default ordering for that model.
@@ -416,11 +440,47 @@ class SQLCompiler(object):
                 results.extend(self.find_ordering_name(item, opts, alias,
                         order, already_seen))
             return results
+        col, alias = self._final_join_removal(col, alias)
+        return [(alias, col, order)]
 
+    def _setup_joins(self, pieces, opts, alias):
+        """
+        A helper method for get_ordering and get_distinct. This method will
+        call query.setup_joins, handle refcounts and then promote the joins.
+
+        Note that get_ordering and get_distinct must produce same target
+        columns on same input, as the prefixes of get_ordering and get_distinct
+        must match. Executing SQL where this is not true is an error.
+        """
+        if not alias:
+            alias = self.query.get_initial_alias()
+        field, target, opts, joins, _, _ = self.query.setup_joins(pieces,
+                opts, alias, False)
+        alias = joins[-1]
+        col = target.column
+        if not field.rel:
+            # To avoid inadvertent trimming of a necessary alias, use the
+            # refcount to show that we are referencing a non-relation field on
+            # the model.
+            self.query.ref_alias(alias)
+
+        # Must use left outer joins for nullable fields and their relations.
+        # Ordering or distinct must not affect the returned set, and INNER
+        # JOINS for nullable fields could do this.
+        self.query.promote_alias_chain(joins,
+            self.query.alias_map[joins[0]][JOIN_TYPE] == self.query.LOUTER)
+        return field, col, alias, joins, opts
+
+    def _final_join_removal(self, col, alias):
+        """
+        A helper method for get_distinct and get_ordering. This method will
+        trim extra not-needed joins from the tail of the join chain.
+
+        This is very similar to what is done in trim_joins, but we will
+        trim LEFT JOINS here. It would be a good idea to consolidate this
+        method and query.trim_joins().
+        """
         if alias:
-            # We have to do the same "final join" optimisation as in
-            # add_filter, since the final column might not otherwise be part of
-            # the select set (so we can't order on it).
             while 1:
                 join = self.query.alias_map[alias]
                 if col != join[RHS_JOIN_COL]:
@@ -428,7 +488,7 @@ class SQLCompiler(object):
                 self.query.unref_alias(alias)
                 alias = join[LHS_ALIAS]
                 col = join[LHS_JOIN_COL]
-        return [(alias, col, order)]
+        return col, alias
 
     def get_from_clause(self):
         """
@@ -438,8 +498,8 @@ class SQLCompiler(object):
         from-clause via a "select".
 
         This should only be called after any SQL construction methods that
-        might change the tables we need. This means the select columns and
+        might change the tables we need. This means the select columns,
-        ordering must be done first.
+        ordering and distinct must be done first.
         """
         result = []
         qn = self.quote_name_unless_alias
@@ -984,6 +1044,7 @@ class SQLAggregateCompiler(SQLCompiler):
         """
         if qn is None:
             qn = self.quote_name_unless_alias
+
         sql = ('SELECT %s FROM (%s) subquery' % (
             ', '.join([
                 aggregate.as_sql(qn, self.connection)

django/db/models/sql/query.py

@@ -127,6 +127,7 @@ class Query(object):
         self.order_by = []
         self.low_mark, self.high_mark = 0, None  # Used for offset/limit
         self.distinct = False
+        self.distinct_fields = []
         self.select_for_update = False
         self.select_for_update_nowait = False
         self.select_related = False
@@ -265,6 +266,7 @@ class Query(object):
         obj.order_by = self.order_by[:]
         obj.low_mark, obj.high_mark = self.low_mark, self.high_mark
         obj.distinct = self.distinct
+        obj.distinct_fields = self.distinct_fields[:]
         obj.select_for_update = self.select_for_update
         obj.select_for_update_nowait = self.select_for_update_nowait
         obj.select_related = self.select_related
@@ -298,6 +300,7 @@ class Query(object):
         else:
             obj.used_aliases = set()
         obj.filter_is_sticky = False
+
         obj.__dict__.update(kwargs)
         if hasattr(obj, '_setup_query'):
             obj._setup_query()
@@ -393,7 +396,7 @@ class Query(object):
         Performs a COUNT() query using the current filter constraints.
         """
         obj = self.clone()
-        if len(self.select) > 1 or self.aggregate_select:
+        if len(self.select) > 1 or self.aggregate_select or (self.distinct and self.distinct_fields):
             # If a select clause exists, then the query has already started to
             # specify the columns that are to be returned.
             # In this case, we need to use a subquery to evaluate the count.
@@ -452,6 +455,8 @@ class Query(object):
                 "Cannot combine queries once a slice has been taken."
         assert self.distinct == rhs.distinct, \
             "Cannot combine a unique query with a non-unique query."
+        assert self.distinct_fields == rhs.distinct_fields, \
+            "Cannot combine queries with different distinct fields."
 
         self.remove_inherited_models()
         # Work out how to relabel the rhs aliases, if necessary.
@@ -674,9 +679,9 @@ class Query(object):
         """ Increases the reference count for this alias. """
         self.alias_refcount[alias] += 1
 
-    def unref_alias(self, alias):
+    def unref_alias(self, alias, amount=1):
         """ Decreases the reference count for this alias. """
-        self.alias_refcount[alias] -= 1
+        self.alias_refcount[alias] -= amount
 
     def promote_alias(self, alias, unconditional=False):
         """
@@ -705,6 +710,15 @@ class Query(object):
             if self.promote_alias(alias, must_promote):
                 must_promote = True
 
+    def reset_refcounts(self, to_counts):
+        """
+        This method will reset reference counts for aliases so that they match
+        the value passed in :param to_counts:.
+        """
+        for alias, cur_refcount in self.alias_refcount.copy().items():
+            unref_amount = cur_refcount - to_counts.get(alias, 0)
+            self.unref_alias(alias, unref_amount)
+
     def promote_unused_aliases(self, initial_refcounts, used_aliases):
         """
         Given a "before" copy of the alias_refcounts dictionary (as
@@ -832,7 +846,8 @@ class Query(object):
     def count_active_tables(self):
         """
         Returns the number of tables in this query with a non-zero reference
-        count.
+        count. Note that after execution, the reference counts are zeroed, so
+        tables added in compiler will not be seen by this method.
         """
         return len([1 for count in self.alias_refcount.itervalues() if count])
 
@@ -1596,6 +1611,13 @@ class Query(object):
         self.select = []
         self.select_fields = []
 
+    def add_distinct_fields(self, *field_names):
+        """
+        Adds and resolves the given fields to the query's "distinct on" clause.
+        """
+        self.distinct_fields = field_names
+        self.distinct = True
+
     def add_fields(self, field_names, allow_m2m=True):
         """
         Adds the given (model) fields to the select set. The field names are

docs/ref/models/querysets.txt

@@ -345,7 +345,7 @@ remain undefined afterward).
 distinct
 ~~~~~~~~
 
-.. method:: distinct()
+.. method:: distinct([*fields])
 
 Returns a new ``QuerySet`` that uses ``SELECT DISTINCT`` in its SQL query. This
 eliminates duplicate rows from the query results.
@@ -374,6 +374,43 @@ query spans multiple tables, it's possible to get duplicate results when a
     :meth:`values()` together, be careful when ordering by fields not in the
     :meth:`values()` call.
 
+.. versionadded:: 1.4
+
+The possibility to pass positional arguments (``*fields``) is new in Django 1.4.
+They are names of fields to which the ``DISTINCT`` should be limited. This
+translates to a ``SELECT DISTINCT ON`` SQL query. A ``DISTINCT ON`` query eliminates
+duplicate rows not by comparing all fields in a row, but by comparing only the given
+fields.
+
+.. note::
+    Note that the ability to specify field names is only available in PostgreSQL.
+
+.. note::
+    When using the ``DISTINCT ON`` functionality it is required that the columns given
+    to :meth:`distinct` match the first :meth:`order_by` columns. For example ``SELECT
+    DISTINCT ON (a)`` gives you the first row for each value in column ``a``. If you
+    don't specify an order, then you'll get some arbitrary row.
+
+Examples::
+
+    >>> Author.objects.distinct()
+    [...]
+
+    >>> Entry.objects.order_by('pub_date').distinct('pub_date')
+    [...]
+
+    >>> Entry.objects.order_by('blog').distinct('blog')
+    [...]
+
+    >>> Entry.objects.order_by('author', 'pub_date').distinct('author', 'pub_date')
+    [...]
+
+    >>> Entry.objects.order_by('blog__name', 'mod_date').distinct('blog__name', 'mod_date')
+    [...]
+
+    >>> Entry.objects.order_by('author', 'pub_date').distinct('author')
+    [...]
+
 values
 ~~~~~~
 

docs/releases/1.4-alpha-1.txt

@@ -507,6 +507,16 @@ Django 1.4 also includes several smaller improvements worth noting:
   ``pickle.HIGHEST_PROTOCOL`` for better compatibility with the other
   cache backends.
 
+* Support in the ORM for generating ``SELECT`` queries containing ``DISTINCT ON``
+
+  The ``distinct()`` ``Queryset`` method now accepts an optional list of model
+  field names. If specified, then the ``DISTINCT`` statement is limited to these
+  fields.  The PostgreSQL is the only of the database backends shipped with
+  Django that supports this new functionality.
+
+  For more details, see the documentation for
+  :meth:`~django.db.models.query.QuerySet.distinct`.
+
 Backwards incompatible changes in 1.4
 =====================================
 

docs/releases/1.4.txt

@@ -498,6 +498,16 @@ Django 1.4 also includes several smaller improvements worth noting:
   ``pickle.HIGHEST_PROTOCOL`` for better compatibility with the other
   cache backends.
 
+* Support in the ORM for generating ``SELECT`` queries containing ``DISTINCT ON``
+
+  The ``distinct()`` ``Queryset`` method now accepts an optional list of model
+  field names. If specified, then the ``DISTINCT`` statement is limited to these
+  fields.  The PostgreSQL is the only of the database backends shipped with
+  Django that supports this new functionality.
+
+  For more details, see the documentation for
+  :meth:`~django.db.models.query.QuerySet.distinct`.
+
 .. _backwards-incompatible-changes-1.4:
 
 Backwards incompatible changes in 1.4

tests/modeltests/distinct_on_fields/__init__.py

@@ -0,0 +1 @@
+#

tests/modeltests/distinct_on_fields/models.py

@@ -0,0 +1,39 @@
+from django.db import models
+
+class Tag(models.Model):
+    name = models.CharField(max_length=10)
+    parent = models.ForeignKey('self', blank=True, null=True,
+            related_name='children')
+
+    class Meta:
+        ordering = ['name']
+
+    def __unicode__(self):
+        return self.name
+
+class Celebrity(models.Model):
+    name = models.CharField("Name", max_length=20)
+    greatest_fan = models.ForeignKey("Fan", null=True, unique=True)
+
+    def __unicode__(self):
+        return self.name
+
+class Fan(models.Model):
+    fan_of = models.ForeignKey(Celebrity)
+
+class Staff(models.Model):
+    id = models.IntegerField(primary_key=True)
+    name = models.CharField(max_length=50)
+    organisation = models.CharField(max_length=100)
+    tags = models.ManyToManyField(Tag, through='StaffTag')
+    coworkers = models.ManyToManyField('self')
+
+    def __unicode__(self):
+        return self.name
+
+class StaffTag(models.Model):
+    staff = models.ForeignKey(Staff)
+    tag = models.ForeignKey(Tag)
+
+    def __unicode__(self):
+        return u"%s -> %s" % (self.tag, self.staff)

tests/modeltests/distinct_on_fields/tests.py

@@ -0,0 +1,116 @@
+from __future__ import absolute_import, with_statement
+
+from django.db.models import Max
+from django.test import TestCase, skipUnlessDBFeature
+
+from .models import Tag, Celebrity, Fan, Staff, StaffTag
+
+class DistinctOnTests(TestCase):
+    def setUp(self):
+        t1 = Tag.objects.create(name='t1')
+        t2 = Tag.objects.create(name='t2', parent=t1)
+        t3 = Tag.objects.create(name='t3', parent=t1)
+        t4 = Tag.objects.create(name='t4', parent=t3)
+        t5 = Tag.objects.create(name='t5', parent=t3)
+
+        p1_o1 = Staff.objects.create(id=1, name="p1", organisation="o1")
+        p2_o1 = Staff.objects.create(id=2, name="p2", organisation="o1")
+        p3_o1 = Staff.objects.create(id=3, name="p3", organisation="o1")
+        p1_o2 = Staff.objects.create(id=4, name="p1", organisation="o2")
+        p1_o1.coworkers.add(p2_o1, p3_o1)
+        StaffTag.objects.create(staff=p1_o1, tag=t1)
+        StaffTag.objects.create(staff=p1_o1, tag=t1)
+
+        celeb1 = Celebrity.objects.create(name="c1")
+        celeb2 = Celebrity.objects.create(name="c2")
+
+        self.fan1 = Fan.objects.create(fan_of=celeb1)
+        self.fan2 = Fan.objects.create(fan_of=celeb1)
+        self.fan3 = Fan.objects.create(fan_of=celeb2)
+
+    @skipUnlessDBFeature('can_distinct_on_fields')
+    def test_basic_distinct_on(self):
+        """QuerySet.distinct('field', ...) works"""
+        # (qset, expected) tuples
+        qsets = (
+            (
+                Staff.objects.distinct().order_by('name'),
+                ['<Staff: p1>', '<Staff: p1>', '<Staff: p2>', '<Staff: p3>'],
+            ),
+            (
+                Staff.objects.distinct('name').order_by('name'),
+                ['<Staff: p1>', '<Staff: p2>', '<Staff: p3>'],
+            ),
+            (
+                Staff.objects.distinct('organisation').order_by('organisation', 'name'),
+                ['<Staff: p1>', '<Staff: p1>'],
+            ),
+            (
+                Staff.objects.distinct('name', 'organisation').order_by('name', 'organisation'),
+                ['<Staff: p1>', '<Staff: p1>', '<Staff: p2>', '<Staff: p3>'],
+            ),
+            (
+                Celebrity.objects.filter(fan__in=[self.fan1, self.fan2, self.fan3]).\
+                    distinct('name').order_by('name'),
+                ['<Celebrity: c1>', '<Celebrity: c2>'],
+            ),
+            # Does combining querysets work?
+            (
+                (Celebrity.objects.filter(fan__in=[self.fan1, self.fan2]).\
+                    distinct('name').order_by('name')
+                |Celebrity.objects.filter(fan__in=[self.fan3]).\
+                    distinct('name').order_by('name')),
+                ['<Celebrity: c1>', '<Celebrity: c2>'],
+            ),
+            (
+                StaffTag.objects.distinct('staff','tag'),
+                ['<StaffTag: t1 -> p1>'],
+            ),
+            (
+                Tag.objects.order_by('parent__pk', 'pk').distinct('parent'),
+                ['<Tag: t2>', '<Tag: t4>', '<Tag: t1>'],
+            ),
+            (
+                StaffTag.objects.select_related('staff').distinct('staff__name').order_by('staff__name'),
+                ['<StaffTag: t1 -> p1>'],
+            ),
+            # Fetch the alphabetically first coworker for each worker
+            (
+                (Staff.objects.distinct('id').order_by('id', 'coworkers__name').
+                               values_list('id', 'coworkers__name')),
+                ["(1, u'p2')", "(2, u'p1')", "(3, u'p1')", "(4, None)"]
+            ),
+        )
+        for qset, expected in qsets:
+            self.assertQuerysetEqual(qset, expected)
+            self.assertEqual(qset.count(), len(expected))
+
+        # Combining queries with different distinct_fields is not allowed.
+        base_qs = Celebrity.objects.all()
+        self.assertRaisesMessage(
+            AssertionError,
+            "Cannot combine queries with different distinct fields.",
+            lambda: (base_qs.distinct('id') & base_qs.distinct('name'))
+        )
+
+        # Test join unreffing
+        c1 = Celebrity.objects.distinct('greatest_fan__id', 'greatest_fan__fan_of')
+        self.assertIn('OUTER JOIN', str(c1.query))
+        c2 = c1.distinct('pk')
+        self.assertNotIn('OUTER JOIN', str(c2.query))
+
+    @skipUnlessDBFeature('can_distinct_on_fields')
+    def test_distinct_not_implemented_checks(self):
+        # distinct + annotate not allowed
+        with self.assertRaises(NotImplementedError):
+            Celebrity.objects.annotate(Max('id')).distinct('id')[0]
+        with self.assertRaises(NotImplementedError):
+            Celebrity.objects.distinct('id').annotate(Max('id'))[0]
+
+        # However this check is done only when the query executes, so you
+        # can use distinct() to remove the fields before execution.
+        Celebrity.objects.distinct('id').annotate(Max('id')).distinct()[0]
+        # distinct + aggregate not allowed
+        with self.assertRaises(NotImplementedError):
+            Celebrity.objects.distinct('id').aggregate(Max('id'))
+

tests/regressiontests/queries/models.py

@@ -209,6 +209,9 @@ class Celebrity(models.Model):
     name = models.CharField("Name", max_length=20)
     greatest_fan = models.ForeignKey("Fan", null=True, unique=True)
 
+    def __unicode__(self):
+        return self.name
+
 class TvChef(Celebrity):
     pass
 
@@ -343,4 +346,3 @@ class OneToOneCategory(models.Model):
 
     def __unicode__(self):
         return "one2one " + self.new_name
-

tests/regressiontests/queries/tests.py

@@ -234,18 +234,22 @@ class Queries1Tests(BaseQuerysetTest):
             ['<Item: four>', '<Item: one>']
         )
 
-    # FIXME: This is difficult to fix and very much an edge case, so punt for
-    # now.  This is related to the order_by() tests for ticket #2253, but the
-    # old bug exhibited itself here (q2 was pulling too many tables into the
-    # combined query with the new ordering, but only because we have evaluated
-    # q2 already).
-    @unittest.expectedFailure
     def test_order_by_tables(self):
         q1 = Item.objects.order_by('name')
         q2 = Item.objects.filter(id=self.i1.id)
         list(q2)
         self.assertEqual(len((q1 & q2).order_by('name').query.tables), 1)
 
+    def test_order_by_join_unref(self):
+        """
+        This test is related to the above one, testing that there aren't
+        old JOINs in the query.
+        """
+        qs = Celebrity.objects.order_by('greatest_fan__fan_of')
+        self.assertIn('OUTER JOIN', str(qs.query))
+        qs = qs.order_by('id')
+        self.assertNotIn('OUTER JOIN', str(qs.query))
+
     def test_tickets_4088_4306(self):
         self.assertQuerysetEqual(
             Report.objects.filter(creator=1001),
@@ -1728,7 +1732,7 @@ class ToFieldTests(TestCase):
 
 
 class ConditionalTests(BaseQuerysetTest):
-    """Tests whose execution depend on dfferent environment conditions like
+    """Tests whose execution depend on different environment conditions like
     Python version or DB backend features"""
 
     def setUp(self):
@@ -1739,6 +1743,7 @@ class ConditionalTests(BaseQuerysetTest):
         t4 = Tag.objects.create(name='t4', parent=t3)
         t5 = Tag.objects.create(name='t5', parent=t3)
 
+
     # In Python 2.6 beta releases, exceptions raised in __len__ are swallowed
     # (Python issue 1242657), so these cases return an empty list, rather than
     # raising an exception. Not a lot we can do about that, unfortunately, due to
@@ -1810,6 +1815,7 @@ class ConditionalTests(BaseQuerysetTest):
             2500
         )
 
+
 class UnionTests(unittest.TestCase):
     """
     Tests for the union of two querysets. Bug #12252.

tests/regressiontests/select_related_regress/tests.py

@@ -40,9 +40,9 @@ class SelectRelatedRegressTests(TestCase):
         self.assertEqual([(c.id, unicode(c.start), unicode(c.end)) for c in connections],
             [(c1.id, u'router/4', u'switch/7'), (c2.id, u'switch/7', u'server/1')])
 
-        # This final query should only join seven tables (port, device and building
+        # This final query should only have seven tables (port, device and building
-        # twice each, plus connection once).
+        # twice each, plus connection once). Thus, 6 joins plus the FROM table.
-        self.assertEqual(connections.query.count_active_tables(), 7)
+        self.assertEqual(str(connections.query).count(" JOIN "), 6)
 
 
     def test_regression_8106(self):