diff --git a/constraint.py b/constraint.py
index 1749194..4ce4e1e 100644
--- a/constraint.py
+++ b/constraint.py
@@ -43,13 +43,22 @@ class ResourceConstraint(object):
 
     def isResourceAllowed(self, resource, task=None):
         if self.referenceType == 'parent':
-            for r in self.referenceValues:
-                m = getattr(r, self.referenceKey)
+            for ref in self.referenceValues:
+                m = getattr(ref, self.referenceKey)
                 if resource in m():
                     return True
             return False
         elif self.referenceType == 'explicit':
             return resource in self.referenceValues
+        elif self.referenceType == 'method':
+            m = getattr(resource, self.referenceKey, None)
+            if m:
+                result = m()
+                if result is None: # method needs additional task parameter
+                    result = m(task=task)
+                return result
+            else:
+                return False
         else:
             return False
 
@@ -57,12 +66,15 @@ class ResourceConstraint(object):
     def getAllowedResources(self, candidates=None, task=None):
         if self.referenceType == 'parent':
             result = []
-            for r in self.referenceValues:
-                m = getattr(r, self.referenceKey)
+            for ref in self.referenceValues:
+                m = getattr(ref, self.referenceKey)
                 result.extend(m())
             return tuple(result)
         elif self.referenceType == 'explicit':
             return tuple(self.referenceValues)
         else:
-            return ()
+            if candidates is None:
+                return None
+            else:
+                return tuple([ r for r in candidates if self.isResourceAllowed(r, task) ])
 
diff --git a/interfaces.py b/interfaces.py
index 0bd5a67..56a89d9 100644
--- a/interfaces.py
+++ b/interfaces.py
@@ -41,24 +41,27 @@ class IResourceConstraint(Interface):
 
     constraintType = TextLine(
         title=u'Constraint Type',
-        description=u'Type of the constraint: select, require, disallow',
+        description=u'Type of the constraint: select, require, allow, disallow',
         default=u'select',
         required=True)
 
     referenceType = TextLine(
         title=u'Reference Type',
         description=u'Type of reference to the resource attribute to check: '
-                     'explicit, parent, type, attribute, property, method',
+                     'explicit, parent, type, method.',
         default=u'explicit',
         required=True)
 
     referenceKey = TextLine(
         title=u'Reference Key',
-        description=u'Key for referencing the resource attribute')
+        description=u'Key for referencing the resource attribute, '
+                     'e.g. method or type name')
 
     referenceValues = List(
         title=u'Reference Values',
-        description=u'Attribute values to check for; may be any kind of object',
+        description=u'Values to check for; usually a list of references to '
+                     'the objects to be selected (referenceType=explicit) '
+                     'or the parent objects (referenceType=parent)',
         value_type=Object(Interface, title=u'Value'),
         unique=True)
 
diff --git a/tests/test_constraint.py b/tests/test_constraint.py
index f06841e..87e64a3 100755
--- a/tests/test_constraint.py
+++ b/tests/test_constraint.py
@@ -53,6 +53,28 @@ class Test(unittest.TestCase):
         rc1.referenceValues = ([t2])
         self.assertEqual(True, rc1.isResourceAllowed(r1))
         self.assertEqual((r1,), rc1.getAllowedResources())
+        rc1.referenceType = 'parent'
+        rc1.referenceKey = 'getAllocatedResources'
+
+    def testRequireMethod(self):
+        rc1 = self.rc1
+        rc1.constraintType = 'require'
+        rc1.referenceType = 'method'
+        rc1.referenceKey = 'isAllowedForTesting'
+        r1 = self.r1
+        t1 = self.t1
+        self.failIf(rc1.isResourceAllowed(r1))
+        Resource.isAllowedForTesting = lambda self: True
+        self.failUnless(rc1.isResourceAllowed(r1))
+        Resource.isAllowedForTesting = lambda self: False
+        self.failIf(rc1.isResourceAllowed(r1))
+        def method(self, task='dummy'):
+            if task == 'dummy': return None # need task keyword parameter
+            return task == t1
+        Resource.isAllowedForTesting = method
+        self.failUnless(rc1.isResourceAllowed(r1, t1))
+        self.failIf(rc1.isResourceAllowed(r1, Task()))
+
 
 
 def test_suite():