Fixed #7314 -- Changed the way extra() bits are handled when QuerySets are merged.

Also added a section to the documentation to indicate why it's probably not a
good idea to rely on this feature for complex stuff. Garbage in, garbage out
applies even to Django code.

Thanks to erik for the test case for this one.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7791 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/sql/query.py

@@ -366,10 +366,21 @@ class Query(object):
                 item.relabel_aliases(change_map)
                 self.select.append(item)
         self.select_fields = rhs.select_fields[:]
-        self.extra_select = rhs.extra_select.copy()
-        self.extra_tables = rhs.extra_tables
-        self.extra_where = rhs.extra_where
-        self.extra_params = rhs.extra_params
+
+        if connector == OR:
+            # It would be nice to be able to handle this, but the queries don't
+            # really make sense (or return consistent value sets). Not worth
+            # the extra complexity when you can write a real query instead.
+            if self.extra_select and rhs.extra_select:
+                raise ValueError("When merging querysets using 'or', you "
+                        "cannot have extra(select=...) on both sides.")
+            if self.extra_where and rhs.extra_where:
+                raise ValueError("When merging querysets using 'or', you "
+                        "cannot have extra(where=...) on both sides.")
+        self.extra_select.update(rhs.extra_select)
+        self.extra_tables += rhs.extra_tables
+        self.extra_where += rhs.extra_where
+        self.extra_params += rhs.extra_params
 
         # Ordering uses the 'rhs' ordering, unless it has none, in which case
         # the current ordering is used.

docs/db-api.txt

@@ -443,6 +443,31 @@ This is roughly equivalent to::
 Note, however, that the first of these will raise ``IndexError`` while the
 second will raise ``DoesNotExist`` if no objects match the given criteria.
 
+Combining QuerySets
+-------------------
+
+If you have two ``QuerySet`` instances that act on the same model, you can
+combine them using ``&`` and ``|`` to get the items that are in both result
+sets or in either results set, respectively. For example::
+
+    Entry.objects.filter(pubdate__gte=date1) & \
+            Entry.objects.filter(headline__startswith="What")
+
+will combine the two queries into a single SQL query. Of course, in this case
+you could have achieved the same result using multiple filters on the same
+``QuerySet``, but sometimes the ability to combine individual ``QuerySet``
+instance is useful.
+
+Be careful, if you are using ``extra()`` to add custom handling to your
+``QuerySet`` however. All the ``extra()`` components are merged and the result
+may or may not make sense. If you are using custom SQL fragments in your
+``extra()`` calls, Django will not inspect these fragments to see if they need
+to be rewritten because of changes in the merged query. So test the effects
+carefully. Also realise that if you are combining two ``QuerySets`` with
+``|``, you cannot use ``extra(select=...)`` or ``extra(where=...)`` on *both*
+``QuerySets``. You can only use those calls on one or the other (Django will
+raise a ``ValueError`` if you try to use this incorrectly).
+
 QuerySet methods that return new QuerySets
 ------------------------------------------
 

tests/regressiontests/extra_regress/models.py

@@ -0,0 +1,55 @@
+import copy
+
+from django.db import models
+from django.db.models.query import Q
+
+
+class RevisionableModel(models.Model):
+    base = models.ForeignKey('self', null=True)
+    title = models.CharField(blank=True, max_length=255)
+
+    def __unicode__(self):
+        return u"%s (%s, %s)" % (self.title, self.id, self.base.id)
+
+    def save(self):
+        super(RevisionableModel, self).save()
+        if not self.base:
+            self.base = self
+            super(RevisionableModel, self).save()
+
+    def new_revision(self):
+        new_revision = copy.copy(self)
+        new_revision.pk = None
+        return new_revision
+
+__test__ = {"API_TESTS": """
+### Regression tests for #7314 and #7372
+
+>>> rm = RevisionableModel.objects.create(title='First Revision')
+>>> rm.pk, rm.base.pk
+(1, 1)
+
+>>> rm2 = rm.new_revision()
+>>> rm2.title = "Second Revision"
+>>> rm2.save()
+>>> print u"%s of %s" % (rm2.title, rm2.base.title)
+Second Revision of First Revision
+
+>>> rm2.pk, rm2.base.pk
+(2, 1)
+
+Queryset to match most recent revision:
+>>> qs = RevisionableModel.objects.extra(where=["%(table)s.id IN (SELECT MAX(rev.id) FROM %(table)s AS rev GROUP BY rev.base_id)" % {'table': RevisionableModel._meta.db_table,}],)
+>>> qs
+[<RevisionableModel: Second Revision (2, 1)>]
+
+Queryset to search for string in title:
+>>> qs2 = RevisionableModel.objects.filter(title__contains="Revision")
+>>> qs2
+[<RevisionableModel: First Revision (1, 1)>, <RevisionableModel: Second Revision (2, 1)>]
+
+Following queryset should return the most recent revision:
+>>> qs & qs2
+[<RevisionableModel: Second Revision (2, 1)>]
+
+"""}