Fixed #12209 -- Made the through attribute on a m2m relation into a property to ensure that the fully resolved through model is always provdided. Thanks to dgouldin for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11736 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/fields/related.py

@@ -586,9 +586,13 @@ class ReverseManyRelatedObjectsDescriptor(object):
     # ReverseManyRelatedObjectsDescriptor instance.
     def __init__(self, m2m_field):
         self.field = m2m_field
+
+    def _through(self):
         # through is provided so that you have easy access to the through
-        # model (Book.authors.through) for inlines, etc.
-        self.through = m2m_field.rel.through
+        # model (Book.authors.through) for inlines, etc. This is done as
+        # a property to ensure that the fully resolved value is returned.
+        return self.field.rel.through
+    through = property(_through)
 
     def __get__(self, instance, instance_type=None):
         if instance is None:

tests/regressiontests/admin_validation/models.py

@@ -26,6 +26,19 @@ class TwoAlbumFKAndAnE(models.Model):
     e = models.CharField(max_length=1)
 
 
+class Author(models.Model):
+    name = models.CharField(max_length=100)
+
+
+class Book(models.Model):
+    name = models.CharField(max_length=100)
+    authors = models.ManyToManyField(Author, through='AuthorsBooks')
+
+
+class AuthorsBooks(models.Model):
+    author = models.ForeignKey(Author)
+    book = models.ForeignKey(Book)
+
 
 __test__ = {'API_TESTS':"""
 
@@ -95,4 +108,18 @@ Exception: <class 'regressiontests.admin_validation.models.TwoAlbumFKAndAnE'> ha
 
 >>> validate_inline(TwoAlbumFKAndAnEInline, None, Album)
 
+# Regression test for #12203 -- If the explicitly provided through model
+# is specified as a string, the admin should still be able use
+# Model.m2m_field.through
+
+>>> class AuthorsInline(admin.TabularInline):
+...     model = Book.authors.through
+
+>>> class BookAdmin(admin.ModelAdmin):
+...     inlines = [AuthorsInline]
+
+# If the through model is still a string (and hasn't been resolved to a model)
+# the validation will fail.
+>>> validate(BookAdmin, Book)
+
 """}