Fixed #8279 -- Multiple many-to-many relations to "self" are now possible.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8721 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/fields/related.py

@@ -844,6 +844,15 @@ class ManyToManyField(RelatedField, Field):
         return smart_unicode(data)
 
     def contribute_to_class(self, cls, name):
+        # To support multiple relations to self, it's useful to have a non-None
+        # related name on symmetrical relations for internal reasons. The
+        # concept doesn't make a lot of sense externally ("you want me to
+        # specify *what* on my non-reversible relation?!"), so we set it up
+        # automatically. The funky name reduces the chance of an accidental
+        # clash.
+        if self.rel.symmetrical and self.rel.related_name is None:
+            self.rel.related_name = "%s_rel_+" % name
+
         super(ManyToManyField, self).contribute_to_class(cls, name)
         # Add the descriptor for the m2m relation
         setattr(cls, self.name, ReverseManyRelatedObjectsDescriptor(self))

tests/regressiontests/m2m_regress/models.py

@@ -0,0 +1,56 @@
+from django.db import models
+
+# No related name is needed here, since symmetrical relations are not
+# explicitly reversible.
+class SelfRefer(models.Model):
+    name = models.CharField(max_length=10)
+    references = models.ManyToManyField('self')
+    related = models.ManyToManyField('self')
+
+    def __unicode__(self):
+        return self.name
+
+class Tag(models.Model):
+    name = models.CharField(max_length=10)
+
+    def __unicode__(self):
+        return self.name
+
+# A related_name is required on one of the ManyToManyField entries here because
+# they are both addressable as reverse relations from Tag.
+class Entry(models.Model):
+    name = models.CharField(max_length=10)
+    topics = models.ManyToManyField(Tag)
+    related = models.ManyToManyField(Tag, related_name="similar")
+
+    def __unicode__(self):
+        return self.name
+
+__test__ = {"regressions": """
+# Multiple m2m references to the same model or a different model must be
+# distinguished when accessing the relations through an instance attribute.
+
+>>> s1 = SelfRefer.objects.create(name='s1')
+>>> s2 = SelfRefer.objects.create(name='s2')
+>>> s3 = SelfRefer.objects.create(name='s3')
+>>> s1.references.add(s2)
+>>> s1.related.add(s3)
+
+>>> e1 = Entry.objects.create(name='e1')
+>>> t1 = Tag.objects.create(name='t1')
+>>> t2 = Tag.objects.create(name='t2')
+>>> e1.topics.add(t1)
+>>> e1.related.add(t2)
+
+>>> s1.references.all()
+[<SelfRefer: s2>]
+>>> s1.related.all()
+[<SelfRefer: s3>]
+
+>>> e1.topics.all()
+[<Tag: t1>]
+>>> e1.related.all()
+[<Tag: t2>]
+
+"""
+}