use empty string instead of None as possible context value to avoid getting an <optgroup> tag in associated widget
class ToTo(EntityType):
permissions = {
'read': ('managers',),
'add': ('managers',),
'update': ('managers',),
'delete': ('managers',),
}
attr = String()
class attr(RelationType):
permissions = {
'read': ('managers', ),
'add': ('managers', RRQLExpression('S bla Y'),),
'delete': ('managers',),
}