[there is an update of this work]

I created a custom taxonomy to use as an index of people mentioned. I wanted it to work nicely as linked data, and so wanted each term in it to refer to the wikidata identifier for the person mentioned. Then I thought, why not get the data for the terms from wikidata?

Brief details

Lots of tutorials on how to set up a custom taxonomy with with custom metadata fields. I worked from this one from smashingmagazine, to get a taxonomy call people, with a custom field for the wikidata id.

Once the wikidata is entered, this code will fetch & parse the data (it’s a work in progress as I add more fields)

<?php
function omni_get_wikidata($wd_id) {
    print('getting wikidata<br />');
    if ('' !== trim( $wd_id) ) {
	    $wd_api_uri = 'https://wikidata.org/entity/'.$wd_id.'.json';
    	$json = file_get_contents( $wd_api_uri );
    	$obj = json_decode($json);
    	return $obj;
    } else {
    	return false;
	}
}

function get_wikidata_value($claim, $datatype) {
	if ( isset( $claim->mainsnak->datavalue->value->$datatype ) ) {
		return $claim->mainsnak->datavalue->value->$datatype;
	} else {
		return false;
	}
}

function omni_get_people_wikidata($term) {
	$term_id = $term->term_id;
    $wd_id = get_term_meta( $term_id, 'wd_id', true );
   	$args = array();
   	$wikidata = omni_get_wikidata($wd_id);
   	if ( $wikidata ) {
    	$wd_name = $wikidata->entities->$wd_id->labels->en->value;
    	$wd_description = $wikidata->entities->$wd_id->descriptions->en->value;
    	$claims = $wikidata->entities->$wd_id->claims;
   		$type = get_wikidata_value($claims->P31[0], 'id');
   		if ( 'Q5' === $type ) {
			if ( isset ($claims->P569[0] ) ) {
				$wd_birth_date = get_wikidata_value($claims->P569[0], 'time');
				print( $wd_birth_date.'<br/>' );
			}
   		} else {
	   		echo(' Warning: that wikidata is not for a human, check the ID. ');
	   		echo(' <br /> ');
   		} 
    	$args['description'] = $wd_description;
    	$args['name'] = $wd_name;
		print_r( $args );print('<br />');
    	update_term_meta( $term_id, 'wd_name', $wd_name );
    	update_term_meta( $term_id, 'wd_description', $wd_description );
    	wp_update_term( $term_id, 'people', $args );
    	
   	} else {
   		echo(' Warning: no wikidata for you, check the Wikidata ID. ');
   	}
}
add_action( 'people_pre_edit_form', 'omni_get_people_wikidata' );
?>

(Note: don’t add this to edited_people hook unless you want along wait while causes itself to be called every time it is called…)

That on its own wasn’t enough. While the name and description of the term were being updated, the values for them displayed in the edit form weren’t updated until the page was refreshed. (Figuring out that it was mostly working took a while.) A bit of javascript inserted into the edit form fixed this:

function omni_taxonomies_edit_fields( $term, $taxonomy ) {
    $wd_id = get_term_meta( $term->term_id, 'wd_id', true );
    $wd_name = get_term_meta( $term->term_id, 'wd_name', true ); 
    $wd_description = get_term_meta( $term->term_id, 'wd_description', true ); 
//JavaScript required so that name and description fields are updated 
    ?>
    <script>
	  var f = document.getElementById("edittag");
	  var n = document.getElementById("name");
  	  var d = document.getElementById("description");
  	  function updateFields() {
  		n.value = "<?php echo($wd_name) ?>";
  		d.innerHTML = "<?php echo($wd_description) ?>";
  	  }

	  f.onsubmit=updateFields();
	</script>
    <tr class="form-field term-group-wrap">
        <th scope="row">
            <label for="wd_id"><?php _e( 'Wikidata ID', 'omniana' ); ?></label>
        </th>
        <td>
            <input type="text" id="wd_id"  name="wd_id" value="<?php echo $wd_id; ?>" />
        </td>
    </tr>
    <?php
}
add_action( 'people_edit_form_fields', 'omni_taxonomies_edit_fields', 10, 2 );