JezK

Edit File: select-advanced.js

( function ( $, rwmb ) {
	'use strict';

// Cache ajax requests: https://github.com/select2/select2/issues/110#issuecomment-419247158
	var cache = {};

/**
	 * Reorder selected values in correct order that they were selected.
	 * @param $select2 jQuery element of the select2.
	 */
	function reorderSelected( $select2 ) {
		var selected = $select2.data( 'selected' );
		if ( ! selected ) {
			return;
		}
		selected.forEach( function ( value ) {
			var option = $select2.children( '[value="' + value + '"]' );
			option.detach();
			$select2.append( option );
		} );
		$select2.trigger( 'change' );
	}

/**
	 * Transform select fields into beautiful dropdown with select2 library.
	 */
	function transform() {
		var $this = $( this ),
			options = $this.data( 'options' );

$this.removeClass( 'select2-hidden-accessible' ).removeAttr( 'data-select2-id' );
		$this.siblings( '.select2-container' ).remove();
		$this.find( 'option' ).removeAttr( 'data-select2-id' );

if ( options.ajax_data ) {
			options.ajax.dataType = 'json';
			options.ajax.data = function( params ) {
				return Object.assign( options.ajax_data, params );
			};
			options.ajax.processResults = function ( response ) {
				var items = response.data.items.map( function( item ) {
					return {
						id: item.value,
						text: item.label,
					}
				} );

var results = {
					results: items
				}
				if ( response.data.hasOwnProperty( 'more' ) ) {
					results.pagination = { more: true };
				}

return results;
			};

options.ajax.transport = function ( params, success, failure ) {
				if ( params.data._type === 'query' ) {
					delete params.data.page;
				}

// Create cache key from ajax params from only neccessary keys to make cache available for multiple fields.
				var data = $.extend( true, {}, params.data );
				delete data.field.id;
				delete data.action;
				if ( ! data.term ) {
					delete data.term;
				}

var key = JSON.stringify( data );
				if ( cache[key] ) {
					success( cache[key] );
					return;
				}

var actions = {
					'post'             : 'rwmb_get_posts',
					'taxonomy'         : 'rwmb_get_terms',
					'taxonomy_advanced': 'rwmb_get_terms',
					'user'             : 'rwmb_get_users'
				};
				params.data.action = actions[ params.data.field.type ];
				params.method = 'POST';

return $.ajax( params ).then( function ( data ) {
					cache[key] = data;
					return data;
				} ).then( success ).fail( failure );
		   };
		}

$this.show().select2( options );

if ( ! $this.attr( 'multiple' ) ) {
			return;
		}

reorderSelected( $this );

/**
		 * Preserve the order that options are selected.
		 * @see https://github.com/select2/select2/issues/3106#issuecomment-255492815
		 */
		$this.on( 'select2:select', function ( event ) {
			var option = $this.children( '[value="' + event.params.data.id + '"]' );
			option.detach();
			$this.append( option ).trigger( 'change' );
		} );
	}

function init( e ) {
		$( e.target ).find( '.rwmb-select_advanced' ).each( transform );
	}

rwmb.$document
		.on( 'mb_ready', init )
		.on( 'clone', '.rwmb-select_advanced', transform );
} )( jQuery, rwmb );