how to create dependent hierarchical select list in Drupal 7?

Any thoughts...

I'm wondering the same thing right now.

I have a nested taxonomy and would like to create some dependant select boxes based on the taxonomy tree... is this possible?

I'm very interested in this as well. Based off what I've researched, I don't think this capability is in core and no contrib modules support this either.

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I'm working on this right now.

First of all I made a taxonomy tree with the hierarchy.

Then I made a module that makes a page with a form as described in the ajax examples module from http://drupal.org/project/examples
With some brainstorming even an novice like me could set the select boxes up into 3 levels with proper ajax calls.

example form elements:

	//Building the actual form elements
    $form['dropdown_first'] = array(
        '#type' => 'select',
        '#title' => 'Boat:',
        '#options' => $options_first,
        '#default_value' => isset($form_state['values']['dropdown_first']) ? $form_state['values']['dropdown_first'] : '',
        // Bind an ajax callback to the change event (which is the default for the
        // select form type) of the first dropdown. It will replace the second
        // dropdown when rebuilt
        '#ajax' => array(
            // When 'event' occurs, Drupal will perform an ajax request in the
            // background. Usually the default value is sufficient (eg. change for
            // select elements), but valid values include any jQuery event,
            // most notably 'mousedown', 'blur', and 'submit'.
            'event' => 'change',
            'callback' => 'taxterms_ajax_callback',
            'wrapper' => 'dropdown_second_replace',
        ),
    );
    
    $form['dropdown_second'] = array(
        '#type' => 'select',
        '#title' => 'Type:',
        '#prefix' => '<div id="dropdown_second_replace">',
        '#suffix' => '</div>',
    	'#options' => $options_second,
        '#default_value' => isset($form_state['values']['dropdown_second']) ? $form_state['values']['dropdown_second'] : '',
    	'#ajax' => array(
            'event' => 'change',
            'callback' => 'taxterms_ajax_callback2',
            'wrapper' => 'dropdown_third_replace',
        ),
    );

It's important to have an '#ajax' array with event and different! callback in every $form['element']

Then I used the function taxonomy_get_nested_tree as described in http://drupal.org/node/831398 (thnx to sylvaticus!) to put the taxonomy data inside my form select boxes.

example first dropdown:

// Get the list of options to populate the first dropdown.    						
    $nested = taxonomy_get_nested_tree(6, $max_depth = NULL, $parent = 0, $parents_index = array(), $depth = 0);
    $options_first = array();
    	$options_first['select'] = 'Select';
  		foreach ($nested as $key => $value) {
    		$options_first[$value->tid] = $value->name;
   		}

Then for the following dropdowns use isset($form_state[...][...]) as seen in $value_dropdown_first in the example form module instead of $parent = 0 to declare the parent and get the right values as seen here:

// Get the list to populate the second dropdown.
	$tree = taxonomy_get_nested_tree(6, 0, isset($form_state['values']['dropdown_first']) ? $form_state['values']['dropdown_first'] : key($options_first), 1);  
	$options_second = array();
		$options_second['select'] = 'Select';
  		foreach ($tree as $key => $value) {
    		$options_second[$value->tid] = $value->name;
       	}

Hopefully this can put you on the way and if not I'm allmost finished with my module so if anybody needs it I can set it online as an example.

Good luck,

Jurgen.

This module is made for the same problem you have identified, i.e making dependent dropdowns for country state and city.

https://www.drupal.org/project/ajax_chain_select