adding restriction around defining counter or container columns as polymorphic keys

cqlengine/models.py

@@ -436,6 +436,9 @@ class ModelMetaClass(type):
 
         polymorphic_column_name, polymorphic_column = polymorphic_columns[0] if polymorphic_columns else (None, None)
 
+        if isinstance(polymorphic_column, (columns.BaseContainerColumn, columns.Counter)):
+            raise ModelDefinitionException('counter and container columns cannot be used for polymorphic keys')
+
         # find polymorphic base class
         polymorphic_base = None
         if is_polymorphic and not is_polymorphic_base:

cqlengine/tests/model/test_polymorphism.py

@@ -62,7 +62,10 @@ class TestPolymorphicClassConstruction(BaseCassEngTestCase):
         assert Base.column_family_name() == M1.column_family_name()
 
     def test_collection_columns_cant_be_polymorphic_keys(self):
-        pass
+        with self.assertRaises(models.ModelDefinitionException):
+            class Base(models.Model):
+                partition = columns.Integer(primary_key=True)
+                type1 = columns.Set(columns.Integer, polymorphic_key=True)
 
 
 class PolyBase(models.Model):