Added clean method to documents for pre validation data cleaning (MongoEngine/mongoengine#60)

docs/guide/document-instances.rst

@@ -38,6 +38,34 @@ already exist, then any changes will be updated atomically.  For example::
 .. seealso::
     :ref:`guide-atomic-updates`
 
+Pre save data validation and cleaning
+-------------------------------------
+MongoEngine allows you to create custom cleaning rules for your documents when
+calling :meth:`~mongoengine.Document.save`.  By providing a custom
+:meth:`~mongoengine.Document.clean` method you can do any pre validation / data
+cleaning.
+
+This might be useful if you want to ensure a default value based on other
+document values for example::
+
+    class Essay(Document):
+        status = StringField(choices=('Published', 'Draft'), required=True)
+        pub_date = DateTimeField()
+
+        def clean(self):
+            """Ensures that only published essays have a `pub_date` and
+            automatically sets the pub_date if published and not set"""
+            if self.status == 'Draft' and self.pub_date is not None:
+                msg = 'Draft entries should not have a publication date.'
+                raise ValidationError(msg)
+            # Set the pub_date for published items if not set.
+            if self.status == 'Published' and self.pub_date is None:
+                self.pub_date = datetime.now()
+
+.. note::
+    Cleaning is only called if validation is turned on and when calling
+:meth:`~mongoengine.Document.save`.
+
 Cascading Saves
 ---------------
 If your document contains :class:`~mongoengine.ReferenceField` or