[1.6.x] Fixed #20820 -- Model inheritance + m2m fixture loading regression

Tests by Tim Graham, report from jeroen.pulles@redslider.net.

Backport of 1ed77e7782 from master

3 files changed, 26 insertions, 0 deletions

diff --git a/tests/fixtures_regress/fixtures/special-article.json b/tests/fixtures_regress/fixtures/special-article.json
new file mode 100644
index 0000000000..435ceb7ca4
--- /dev/null
+++ b/tests/fixtures_regress/fixtures/special-article.json
@@ -0,0 +1,10 @@
+[
+    {
+        "pk": 1,
+        "model": "fixtures_regress.specialarticle",
+        "fields": {
+            "title": "Article Title 1",
+            "channels": []
+        }
+    }
+]
diff --git a/tests/fixtures_regress/models.py b/tests/fixtures_regress/models.py
index 1cc30d2e51..7ba069ad08 100644
--- a/tests/fixtures_regress/models.py
+++ b/tests/fixtures_regress/models.py
@@ -70,6 +70,11 @@ class Article(models.Model):
         ordering = ('id',)
 
 
+# Subclass of a model with a ManyToManyField for test_ticket_20820
+class SpecialArticle(Article):
+    pass
+
+
 # Models to regression test #11428
 @python_2_unicode_compatible
 class Widget(models.Model):
diff --git a/tests/fixtures_regress/tests.py b/tests/fixtures_regress/tests.py
index a6bad5716d..1d0fb9e724 100644
--- a/tests/fixtures_regress/tests.py
+++ b/tests/fixtures_regress/tests.py
@@ -431,6 +431,17 @@ class TestFixtures(TestCase):
         self.assertTrue("No fixture 'this_fixture_doesnt_exist' in" in
             force_text(stdout_output.getvalue()))
 
+    def test_ticket_20820(self):
+        """
+        Regression for ticket #20820 -- loaddata on a model that inherits
+        from a model with a M2M shouldn't blow up.
+        """
+        management.call_command(
+            'loaddata',
+            'special-article.json',
+            verbosity=0,
+        )
+
 
 class NaturalKeyFixtureTests(TestCase):