Improved custom object identifier test

This provides an example for implementors and ensures that failing to
use the custom class would cause a test failure.

test_haystack/core/custom_identifier.py

@@ -2,10 +2,16 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
+import hashlib
 
 def get_identifier_method(key):
     """
     Custom get_identifier method used for testing the
     setting HAYSTACK_IDENTIFIER_MODULE
     """
-    return key
+
+    if hasattr(key, 'get_custom_haystack_id'):
+        return key.get_custom_haystack_id()
+    else:
+        key_bytes = key.encode('utf-8')
+        return hashlib.md5(key_bytes).hexdigest()

test_haystack/test_utils.py

@@ -30,8 +30,20 @@ def test_get_identifier(self):
 
     @override_settings(HAYSTACK_IDENTIFIER_METHOD='test_haystack.core.custom_identifier.get_identifier_method')
     def test_haystack_identifier_method(self):
+        # The custom implementation returns the MD-5 hash of the key value by
+        # default:
         get_identifier = _lookup_identifier_method()
-        self.assertEqual(get_identifier('a.b.c'), 'a.b.c')
+        self.assertEqual(get_identifier('a.b.c'),
+                         '553f764f7b436175c0387e22b4a19213')
+
+        # … but it also supports a custom override mechanism which would
+        # definitely fail with the default implementation:
+        class custom_id_class(object):
+            def get_custom_haystack_id(self):
+                return 'CUSTOM'
+
+        self.assertEqual(get_identifier(custom_id_class()),
+                         'CUSTOM')
 
     @override_settings(HAYSTACK_IDENTIFIER_METHOD='test_haystack.core.custom_identifier.not_there')
     def test_haystack_identifier_method_bad_path(self):