gistfile1.txt

diff --git a/ming/orm/property.py b/ming/orm/property.py
index 2ad5dcf..296d8c7 100644
--- a/ming/orm/property.py
+++ b/ming/orm/property.py
@@ -1,5 +1,6 @@
-from ming.base import Field
+from ming.base import Field, Object
 from ming.utils import LazyProperty
+from ming.schema import SchemaItem
 from .base import session, state, mapper, lookup_class
 from .icollection import InstrumentedList
 
@@ -60,6 +61,9 @@ class FieldProperty(ORMProperty):
             return getattr(st.document, self.name)
 
     def __set__(self, instance, value):
+        si = SchemaItem.make(self.field_type, *self.args, **self.kwargs)
+        si.validate(value)
+
         st = state(instance)
         st.soil()
         st.document[self.name] = value
diff --git a/ming/schema.py b/ming/schema.py
index f18c19b..f9fd0e1 100644
--- a/ming/schema.py
+++ b/ming/schema.py
@@ -200,6 +200,8 @@ class FancySchemaItem(SchemaItem):
             self.__class__.__name__, self.required)
 
     def validate(self, value, **kw):
+        if value is None and self.required:
+            value = Missing
         if value is Missing:
             if self.required:
                 raise Invalid('Missing field', value, None)
diff --git a/ming/tests/test_orm.py b/ming/tests/test_orm.py
index 0282f80..eb1511c 100644
--- a/ming/tests/test_orm.py
+++ b/ming/tests/test_orm.py
@@ -126,6 +126,15 @@ class TestBasicMapping(TestCase):
         q = self.Basic.query.find()
         self.assertEqual(q.count(), 0)
 
+    def test_validate_upon_set(self):
+        doc = self.Basic(a=1)
+        try:
+            doc.a = 'foobar'
+        except S.Invalid:
+            pass
+        else:
+            raise AssertionError('Invalid exception expected')
+
     def test_query(self):
         doc = self.Basic(a=1, b=[2,3], c=dict(d=4, e=5))
         self.session.flush()
diff --git a/ming/tests/test_schema.py b/ming/tests/test_schema.py
index ee43d17..dccdc50 100644
--- a/ming/tests/test_schema.py
+++ b/ming/tests/test_schema.py
@@ -105,6 +105,10 @@ class TestSchemaItem(TestCase):
         si = S.SchemaItem.make(dict(version=4))
         self.assertEqual(si.validate(dict(version=4)), dict(version=4))
         self.assertRaises(S.Invalid, si.validate, dict(version=3))
+
+    def test_required_none(self):
+        si = S.SchemaItem.make(int, required=True)
+        self.assertRaises(S.Invalid, si.validate, None)
     
     def test_missing(self):
         self.assertEqual(repr(S.Missing), '<Missing>')