In an object_select_tag, the toString() method is automacally called to display the object in the list.

Sometimes, you can't use the same method for diffrent list on the same objects, so you have to specify wich method to call.

So this is the tip : 'text_method' => 'theMethodToCall'

object_select_tag($my_object, 'myMethod', array (
  'related_class' => 'MyObjectClass',
  'text_method' => 'getCompleteDescription'
))