Added test example of updating an embedded field

Closes #139

tests/document.py

@@ -588,6 +588,34 @@ class DocumentTest(unittest.TestCase):
         # Ensure that the 'details' embedded object saved correctly
         self.assertEqual(employee_obj['details']['position'], 'Developer')
 
+    def test_updating_an_embedded_document(self):
+        """Ensure that a document with an embedded document field may be
+        saved in the database.
+        """
+        class EmployeeDetails(EmbeddedDocument):
+            position = StringField()
+
+        class Employee(self.Person):
+            salary = IntField()
+            details = EmbeddedDocumentField(EmployeeDetails)
+
+        # Create employee object and save it to the database
+        employee = Employee(name='Test Employee', age=50, salary=20000)
+        employee.details = EmployeeDetails(position='Developer')
+        employee.save()
+
+        # Test updating an embedded document
+        promoted_employee = Employee.objects.get(name='Test Employee')
+        promoted_employee.details.position = 'Senior Developer'
+        promoted_employee.save()
+
+        collection = self.db[self.Person._meta['collection']]
+        employee_obj = collection.find_one({'name': 'Test Employee'})
+        self.assertEqual(employee_obj['name'], 'Test Employee')
+        self.assertEqual(employee_obj['age'], 50)
+        # Ensure that the 'details' embedded object saved correctly
+        self.assertEqual(employee_obj['details']['position'], 'Senior Developer')
+
     def test_save_reference(self):
         """Ensure that a document reference field may be saved in the database.
         """