diff --git a/composer/schema/browser/schema_macros.pt b/composer/schema/browser/schema_macros.pt
index 6a20e69..541c9ab 100755
--- a/composer/schema/browser/schema_macros.pt
+++ b/composer/schema/browser/schema_macros.pt
@@ -162,7 +162,7 @@
                            style python:
                                 'width: %s' % (width and str(width)+'px' or '');">
       <option tal:repeat="item python:
-                    field.getVocabularyItems(instance.context, request)"
+                    field.getVocabularyItems(instance, request)"
               tal:content="item/title"
               tal:attributes="value item/token;
                               selected python:
diff --git a/composer/schema/field.py b/composer/schema/field.py
index 1f2ecab..593691e 100644
--- a/composer/schema/field.py
+++ b/composer/schema/field.py
@@ -129,7 +129,10 @@ class Field(Component):
     def getTitleValue(self):
         return self.title or self.name
 
-    def getVocabularyItems(self, context=None, request=None):
+    def getVocabularyItems(self, instance=None, request=None):
+        context = None
+        if instance is not None:
+            context = instance.context
         voc = (self.vocabulary or '')
         if isinstance(voc, basestring):
             terms = self.getVocabularyTerms(voc, context, request)
@@ -138,7 +141,7 @@ class Field(Component):
             voc = voc.splitlines()
             return [dict(token=t, title=t) for t in voc if t.strip()]
         elif IContextSourceBinder.providedBy(voc):
-            source = voc(context)
+            source = voc(instance)
             terms = component.queryMultiAdapter((source, request), ITerms)
             if terms is not None:
                 termsList = [terms.getTerm(value) for value in source]
@@ -395,7 +398,7 @@ class CheckBoxesFieldInstance(ListFieldInstance):
 class DropdownFieldInstance(FieldInstance):
 
     def display(self, value):
-        items = self.context.getVocabularyItems(self.clientContext, self.request)
+        items = self.context.getVocabularyItems(self.clientInstance, self.request)
         for item in items:
             if item['token'] == value:
                 return item['title']
diff --git a/composer/schema/grid/field.py b/composer/schema/grid/field.py
index b2eefc9..95195f0 100644
--- a/composer/schema/grid/field.py
+++ b/composer/schema/grid/field.py
@@ -120,7 +120,8 @@ class GridFieldInstance(ListFieldInstance):
             if fi.default is not None:
                 if value == fi.default:
                     continue
-            item[fi.name] = value
+            if value:
+                item[fi.name] = value
         return item
 
 
@@ -189,7 +190,7 @@ class KeyTableFieldInstance(RecordsFieldInstance):
         for row in value:
             item = self.unmarshallRow(row)
             if item:
-                result[item.pop(self.keyName)] = [item.get(name)
+                result[item.pop(self.keyName)] = [item.get(name) or u''
                                                   for name in self.dataNames]
         return result
 
diff --git a/util/iterate.py b/util/iterate.py
new file mode 100644
index 0000000..6df52a6
--- /dev/null
+++ b/util/iterate.py
@@ -0,0 +1,55 @@
+#
+#  Copyright (c) 2012 Helmut Merz helmutm@cy55.de
+#
+#  This program is free software; you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation; either version 2 of the License, or
+#  (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software
+#  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+
+"""
+Iterator and generator utilities.
+"""
+
+from itertools import islice
+
+
+class BatchIterator(object):
+
+    def __init__(self, data, limit=20, start=0):
+        self.data = iter(data)
+        self.limit = limit
+        self.start = start
+        self.count = 0
+        self.batch = 0
+        self.exhausted = False
+
+    def __iter__(self):
+        return self
+
+    def next(self):
+        if self.count >= (self.batch + 1) * self.limit:
+            raise StopIteration
+        if self.start:
+            for i in islice(self.data, 0, self.start*self.limit):
+                pass
+            self.start = 0
+        self.count += 1
+        try:
+            return self.data.next()
+        except StopIteration:
+            self.exhausted = True
+            raise
+
+    def advance(self, batches=1):
+        self.batch += batches
+        return not self.exhausted
diff --git a/util/iterate.txt b/util/iterate.txt
new file mode 100644
index 0000000..4acacb8
--- /dev/null
+++ b/util/iterate.txt
@@ -0,0 +1,54 @@
+================================
+Iterator and Generator Utilities
+================================
+
+
+Batch Iterator
+==============
+
+A batch iterator only provides a limited number of items in one
+series of access steps.
+
+  >>> from cybertools.util.iterate import BatchIterator
+
+We create a BatchIterator upon a base iterator. The BatchIterator
+only gives us a limited portion of the values provided by the base
+iterator.
+
+  >>> it = BatchIterator(xrange(30))
+  >>> list(it)
+  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
+  >>> list(it)
+  []
+
+Now we advance to the next batch. The return value tells us that the
+base iterator is not exhausted yet.
+
+  >>> it.advance()
+  True
+  >>> list(it)
+  [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]
+
+Advancing would not help if the base iterator is exhausted.
+
+  >>> it.advance()
+  False
+  >>> list(it)
+  []
+
+We can also immediately start at the second batch by providing the ``start``
+argument to the BatchIterator constructor.
+
+  >>> it = BatchIterator(xrange(30), start=1)
+  >>> list(it)
+  [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]
+
+We can use another limit (i.e. the batch size) via the BatchIterator constructor.
+
+  >>> it = BatchIterator(xrange(30), start=1, limit=8)
+  >>> list(it)
+  [8, 9, 10, 11, 12, 13, 14, 15]
+  >>> it.advance()
+  True
+  >>> list(it)
+  [16, 17, 18, 19, 20, 21, 22, 23]
diff --git a/util/tests.py b/util/tests.py
index a5ef08e..dcf2605 100755
--- a/util/tests.py
+++ b/util/tests.py
@@ -25,6 +25,7 @@ def test_suite():
         doctest.DocFileSuite('defer.txt', optionflags=flags),
         doctest.DocFileSuite('format.txt', optionflags=flags),
         doctest.DocFileSuite('html.txt', optionflags=flags),
+        doctest.DocFileSuite('iterate.txt', optionflags=flags),
         doctest.DocFileSuite('multikey.txt', optionflags=flags),
         doctest.DocFileSuite('property.txt', optionflags=flags),
         doctest.DocFileSuite('json.txt', optionflags=flags),