def find_permission(permissions, permission_type, object_id=None, namespace
    =None):
    """Given a queryset of permissions, filters depending on the permission
    type, and optionally an object id and namespace."""
    if object_id is not None:
        return permissions.filter(type=permission_type, object_id=object_id,
            namespace=namespace)
    return permissions.filter(type=permission_type)
