Add create_from_super method and test

Author: JCoxwell

src/polymorphic/query.py

@@ -531,3 +531,35 @@ def get_real_instances(self, base_result_objects=None):
             return olist
         clist = PolymorphicQuerySet._p_list_class(olist)
         return clist
+
+    def create_from_super(self, obj, **kwargs):
+        """Creates an instance of self.model (cls) from existing super class.
+        The new subclass will be the same object with same database id
+        and data as obj, but will be an instance of cls.
+
+        obj must be an instance of the direct superclass of cls.
+        kwargs should contain all required fields of the subclass (cls).
+
+        returns obj as an instance of cls.
+        """
+        cls = self.model
+        import inspect
+
+        scls = inspect.getmro(cls)[1]
+        if scls is not type(obj):
+            raise Exception(
+                "create_from_super can only be used if obj is one level of inheritance up from cls"
+            )
+        ptr = "{}_ptr_id".format(scls.__name__.lower())
+        kwargs[ptr] = obj.id
+        # create the new base class with only fields that apply to  it.
+        nobj = cls(**kwargs)
+        nobj.save_base(raw=True)
+        # force update the content type, but first we need to
+        # retrieve a clean copy from the db to fill in the null
+        # fields otherwise they would be overwritten.
+        nobj = cls.objects.get(pk=obj.pk)
+        nobj.polymorphic_ctype = ContentType.objects.get_for_model(cls)
+        nobj.save()
+
+        return nobj.get_real_instance()  # cast to cls

src/polymorphic/tests/test_recasting.py

@@ -0,0 +1,24 @@
+from django.test import TransactionTestCase
+from polymorphic.tests.models import Model2A, Model2B, Model2C, Model2D
+
+
+class PolymorphicTests(TransactionTestCase):
+    def test_create_from_super(self):
+        # run create test 3 times because initial implementation
+        # would fail after first success.
+        for i in range(3):
+            mc = Model2C.objects.create(
+                field1="C1{}".format(i), field2="C2{}".format(i), field3="C3{}".format(i)
+            )
+            mc.save()
+            field4 = "D4{}".format(i)
+            md = Model2D.objects.create_from_super(mc, field4=field4)
+            self.assertEqual(mc.id, md.id)
+            self.assertEqual(mc.field1, md.field1)
+            self.assertEqual(mc.field2, md.field2)
+            self.assertEqual(mc.field3, md.field3)
+            self.assertEqual(md.field4, field4)
+        ma = Model2A.objects.create(field1="A1e")
+        self.assertRaises(Exception, Model2D.objects.create_from_super, ma, field4="D4e")
+        mb = Model2B.objects.create(field1="B1e", field2="B2e")
+        self.assertRaises(Exception, Model2D.objects.create_from_super, mb, field4="D4e")