more work on coverage

tests/test_common.py

@@ -0,0 +1,15 @@
+import unittest
+
+from mongoengine.common import _import_class
+from mongoengine import Document
+
+
+class TestCommon(unittest.TestCase):
+
+    def test__import_class(self):
+        doc_cls = _import_class("Document")
+        self.assertIs(doc_cls, Document)
+
+    def test__import_class_raise_if_not_known(self):
+        with self.assertRaises(ValueError):
+            _import_class("UnknownClass")