How to only list the child terms of a taxonomy and not their parents?

The question:

I’m creating an age select menu in the admin, populated from a taxonomy of age. The taxonomy is hierarchical as follows:

  • 18-25 (parent, ID 183)
    • 18 (child)
    • 19
    • 20
    • 21
    • 22
    • 23
    • 24
    • 25
  • 26-30 (parent, ID 184)
    • 26
    • 27
    • 28
    • 29
    • 30

I would like to only list the children (18, 19 etc) and not the parents (18-25, 26-30) etc. Currently I am using get_terms with the parent argument, but it doesn’t accept more than 1 parent ID. Here’s what I have so far, which shows the children from 18-25.

    $ages = get_terms( 'age', array(
        'hide_empty' => 0,
        'parent' => '183',
    ));

Here’s what I want it to do, but isn’t supported. I have also tried it with an array but it doesn’t work either.

    $ages = get_terms( 'age', array(
        'hide_empty' => 0,
        'parent' => '183,184',
    ));

I see there is a get_term_children function but I’m unsure of how to use this either as it looks like it only accepts one value also. Eg:
In this example it would build an unordered list but I could modify for select menu.

<?php
    $termID = 183;
    $taxonomyName = "age";
    $termchildren = get_term_children( $termID, $taxonomyName );

    echo '<ul>';
    foreach ($termchildren as $child) {
    $term = get_term_by( 'id', $child, $taxonomyName );
    echo '<li><a href="' . get_term_link( $term->name, $taxonomyName ) . '" rel="nofollow noreferrer noopener">' . $term->name . '</a></li>';
    }
    echo '</ul>';
?> 

The Solutions:

Below are the methods you can try. The first solution is probably the best. Try others if the first one doesn’t work. Senior developers aren’t just copying/pasting – they read the methods carefully & apply them wisely to each case.

Method 1

This should work for you:

$taxonomyName = "age";
//This gets top layer terms only.  This is done by setting parent to 0.  
$parent_terms = get_terms( $taxonomyName, array( 'parent' => 0, 'orderby' => 'slug', 'hide_empty' => false ) );   
echo '<ul>';
foreach ( $parent_terms as $pterm ) {
    //Get the Child terms
    $terms = get_terms( $taxonomyName, array( 'parent' => $pterm->term_id, 'orderby' => 'slug', 'hide_empty' => false ) );
    foreach ( $terms as $term ) {
        echo '<li><a href="' . get_term_link( $term ) . '" rel="nofollow noreferrer noopener" rel="nofollow noreferrer noopener">' . $term->name . '</a></li>';   
    }
}
echo '</ul>';

Method 2

You could also do:

$terms = get_terms($taxonomyName);
foreach($terms as $term) {
    if ($term->parent != 0) { // avoid parent categories
        //your instructions here
    }
}

I’ve noted that parent have “parent” field equal to 0, and a child have his parent id in it.

Method 3

We can exclude the top level parents by filtering them out by using the terms_clauses filter to alter the SQL query before it executes. This way we do not need to skip parents in the final foreach loop as they are not in the returned array of terms, this saves us unnecessary work and coding

You can try the following:

add_filter( 'terms_clauses', function (  $pieces, $taxonomies, $args )
{
    // Check if our custom arguments is set and set to 1, if not bail
    if (    !isset( $args['wpse_exclude_top'] ) 
         || 1 !== $args['wpse_exclude_top']
    )
        return $pieces;

    // Everything checks out, lets remove parents
    $pieces['where'] .= ' AND tt.parent > 0';

    return $pieces;
}, 10, 3 );

To exclude top level parents, we can now pass 'wpse_exclude_top' => 1 with our array of arguments. The new wpse_exclude_top parameter is supported by the filter above

$terms = get_terms( 'category', ['wpse_exclude_top' => 1] );
if (    $terms
     && !is_wp_error( $terms )
) {
    echo '<ul>';
        foreach ($terms as $term) {
            echo '<li><a href="' . get_term_link( $term ) . '" rel="nofollow noreferrer noopener" rel="nofollow noreferrer noopener">' . $term->name . '</a></li>';  
        }
    echo '</ul>';
}

Just a note, get_term_link() do no not accept the term name, only, slug, ID or the complete term object. For performance, always always pass the term object to get_term_link() if the term object is available (as in this case)

Method 4

Why can’t you just set the childless argument to true?

$ages = get_terms( array(
    'taxonomy' => 'age',
    'childless' => true
    )
);

Method 5

If you display multiple parent’s child, you can try this. Display mention term ids child term.

$termIds = array(367, 366, 365, 364, 363, 362);
$taxonomyName = "age";

$args = array(
    'orderby'           => 'term_id', 
    'order'             => 'DESC',                      
    'hide_empty'        => false,                       
    'childless'         => false,
); 

$terms = get_terms( $taxonomyName, $args );

if ( ! empty( $terms ) && ! is_wp_error( $terms ) ){
$inc = 1;
foreach ( $terms as $term ) {                           
 if (in_array($term->parent, $termIds)) {
  echo '<option value="'.$term->term_id.'"><font><font>'.$term->name.'</font></font></option>';
    }
  }
}


All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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