get field value from the actual local roles (#4492)

src/collective/z3cform/rolefield/field.py

@@ -57,6 +57,20 @@ class LocalRolesToPrincipalsDataManager(AttributeField):
     """A data manager which set local roles when saving the field."""
     adapts(Interface, ILocalRolesToPrincipals)
 
+    def get(self):
+        # get value from the actual local roles, in case it has been modified
+        # by an external process
+        roles_to_assign = frozenset(self.field.roles_to_assign)
+
+        value = []
+        for local_role in self.context.get_local_roles():
+            # a local_role is like ('Administrators', (u'Contributor', u'Reviewer'))
+            principal = local_role[0]
+            local_roles = frozenset(local_role[1])
+            if roles_to_assign.issubset(local_roles):
+                value.append(principal)
+        return value
+
     def set(self, value):
         """See z3c.form.interfaces.IDataManager"""
         # set local roles before setting the value so we still have access to the old value