fix-#734: set attribute to None does not work (at least for fields wi…

…th default values). Solves #735 as well

mongoengine/base/fields.py

@@ -37,7 +37,7 @@ class BaseField(object):
     def __init__(self, db_field=None, name=None, required=False, default=None,
                  unique=False, unique_with=None, primary_key=False,
                  validation=None, choices=None, verbose_name=None,
-                 help_text=None):
+                 help_text=None, null=False):
         """
         :param db_field: The database field to store this field in
             (defaults to the name of the field)
@@ -60,6 +60,8 @@ def __init__(self, db_field=None, name=None, required=False, default=None,
             model forms from the document model.
         :param help_text: (optional) The help text for this field and is often
             used when generating model forms from the document model.
+        :param null: (optional) Is the field value can be null. If no and there is a default value
+            then the default value is set
         """
         self.db_field = (db_field or name) if not primary_key else '_id'
 
@@ -75,6 +77,7 @@ def __init__(self, db_field=None, name=None, required=False, default=None,
         self.choices = choices
         self.verbose_name = verbose_name
         self.help_text = help_text
+        self.null = null
 
         # Adjust the appropriate creation counter, and save our local copy.
         if self.db_field == '_id':
@@ -100,10 +103,13 @@ def __set__(self, instance, value):
 
         # If setting to None and theres a default
         # Then set the value to the default value
-        if value is None and self.default is not None:
-            value = self.default
-            if callable(value):
-                value = value()
+        if value is None:
+            if self.null:
+                value = None
+            elif self.default is not None:
+                value = self.default
+                if callable(value):
+                    value = value()
 
         if instance._initialised:
             try:

tests/document/instance.py

@@ -2643,5 +2643,26 @@ class Person(Document):
         self.assertEquals(p4.height, 189)
         self.assertEquals(Person.objects(height=189).count(), 1)
 
+    def test_null_field(self):
+        # 734
+        class User(Document):
+            name = StringField()
+            height = IntField(default=184, null=True)
+        User.objects.delete()
+        u = User(name='user')
+        u.save()
+        u_from_db = User.objects.get(name='user')
+        u_from_db.height = None
+        u_from_db.save()
+        self.assertEquals(u_from_db.height, None)
+
+        # 735
+        User.objects.delete()
+        u = User(name='user')
+        u.save()
+        User.objects(name='user').update_one(set__height=None, upsert=True)
+        u_from_db = User.objects.get(name='user')
+        self.assertEquals(u_from_db.height, None)
+
 if __name__ == '__main__':
     unittest.main()