I had two separate non-hierarchical taxonomies for Manufacturers and Models but decided to make them one single instead of forging a relationship between them as separate taxonomies altogether…

So now I have them in one, make_model taxonomy, and eventually intend to get rid of their default UI, once I have a custom working alternative; because for one thing I only want them to be 1 level deep, that is, excluding the top level — so that top level would be Manufacturers, and the first sub-level would be Models per, and go no deeper. I don’t know how to set a maximum.

First what I need to do is create a meta box in my CPT edit screen, with a select field for Manufacturer, and then a Model field that shows conditionally if any manufacturer is selected, its children, allowing for a selection to be made finally of the Model.

Another thing is that I need a default null value for both of these fields in question. The Model is to be a required field, and for one to be selected, a Manufacturer must be selected, so just the first one cannot be selected by default; default must be no selected value, and upon changing the value of the Manufacturer, the value of the Model should reset to null as well.

Here’s my manufacturer field:

piklist('field', array(
  'type' => 'select'
  ,'scope' => 'taxonomy'
  ,'field' => 'manufacturer'
  ,'label' => 'Manufacturer'
  ,'choices' => piklist(
      get_terms('make_model', array(
        'hide_empty' => false
        ,'parent' => 0
      ))
      ,array(
        'term_id'
        ,'name'
      )
  )
));

Model I figured would somehow go something like this:

piklist('field', array(
  'type' => 'select'
  ,'scope' => 'taxonomy'
  ,'field' => 'model'
  ,'label' => 'Model'
  ,'choices' => piklist(
      get_terms('make_model', array(
        'hide_empty' => false
        //,'child_of' => 
      ))
      ,array(
        'term_id'
        ,'name'
      )
  )
));

utilizing the conditions parameters somehow like here https://piklist.com/user-guide/docs/fields/field-parameters/conditions/ except I don’t quite see how I can tie it all together.

Ideas?

Thanks.