title: Xcode bindings: can not add object to NSArrayController
body: <p><P>
  I have an entity <strong>Axis</strong> with relationship [to many] to entity <strong>AxisValues</strong>.<P>
  And I try to add <strong>AxisValue</strong> object to selected object <strong>Axis</strong>. I bind one NSArrayController <em>Axes</em> to ManagedObjectContext, entity <strong>Axis</strong>. The second NSArrayController <em>Selected Axis Values</em> with entity <strong>AxisValue</strong> is bind by ContentSet to <em>Axes</em>, ControllerKey <em>selection</em>, ModelKeyPath <em>axisValues</em>.<P>
  In interface it works nice, in a NSTableView I have proper values. But I cannot add or remove <strong>AxisValue</strong> objects by action <strong>add:</strong> or <strong>remove:</strong> connected to <em>Selected Axis Values</em> controller.<P>
  How to bind <em>Selected Axis Values</em> to have possibility to add and remove objects by actions <strong>add:</strong> and <strong>remove:</strong>? </p>