<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_placeholder_field_names(self):
 """ Returns a list with the names of all PlaceholderFields. """

        return [field.name for field in self._meta.fields if field.get_internal_type() == 'PlaceholderField']