Added get_base_polymorphic_model() to detect the common base class for a polymorphic model.

(cherry picked from commit 04d4181e17)

polymorphic/tests/test_utils.py

@@ -1,9 +1,9 @@
 from unittest import TestCase
 
-from polymorphic.models import PolymorphicTypeUndefined
-from polymorphic.tests import Model2A, Model2B, Model2C, Model2D
+from polymorphic.models import PolymorphicTypeUndefined, PolymorphicModel
+from polymorphic.tests import Model2A, Model2B, Model2C, Model2D, Enhance_Inherit, Enhance_Base
 from polymorphic.tests.test_orm import qrepr
-from polymorphic.utils import sort_by_subclass, reset_polymorphic_ctype
+from polymorphic.utils import reset_polymorphic_ctype, sort_by_subclass, get_base_polymorphic_model
 
 
 class UtilsTests(TestCase):
@@ -35,3 +35,39 @@ class UtilsTests(TestCase):
             ' <Model2B: id 3, field1 (CharField), field2 (CharField)>,'
             ' <Model2B: id 4, field1 (CharField), field2 (CharField)>]'
         ))
+
+    def test_get_base_polymorphic_model(self):
+        """
+        Test that finding the base polymorphic model works.
+        """
+        # Finds the base from every level (including lowest)
+        self.assertIs(get_base_polymorphic_model(Model2D), Model2A)
+        self.assertIs(get_base_polymorphic_model(Model2C), Model2A)
+        self.assertIs(get_base_polymorphic_model(Model2B), Model2A)
+        self.assertIs(get_base_polymorphic_model(Model2A), Model2A)
+
+        # Properly handles multiple inheritance
+        self.assertIs(get_base_polymorphic_model(Enhance_Inherit), Enhance_Base)
+
+        # Ignores PolymorphicModel itself.
+        self.assertIs(get_base_polymorphic_model(PolymorphicModel), None)
+
+    def test_get_base_polymorphic_model_skip_abstract(self):
+        """
+        Skipping abstract models that can't be used for querying.
+        """
+        class A(PolymorphicModel):
+            class Meta:
+                abstract = True
+
+        class B(A):
+            pass
+
+        class C(B):
+            pass
+
+        self.assertIs(get_base_polymorphic_model(A), None)
+        self.assertIs(get_base_polymorphic_model(B), B)
+        self.assertIs(get_base_polymorphic_model(C), B)
+
+        self.assertIs(get_base_polymorphic_model(C, allow_abstract=True), A)