Input renderer for Magento category attribute that appends JavaScript helper code to the element.

Blogcategory.php

class MyPackage_Wordpress_Block_Catalog_Category_Helper_Blogcategory extends Varien_Data_Form_Element_Select {
    /**
     * Attaches javascript helper library to element output.
     *
     * @return mixed|string The final element HTML output
     */
    public function getAfterElementHtml() {
        $html=parent::getAfterElementHtml();
        $html.=
            '<script language="text/javascript">
                /**
                 * Creates dependency of categories drop-down on blog drop-down by
                 * loading categories based on blog selection.
                 *
                 * @todo [low] Move code into standalone JS file and initialize with parameters.
                 */
                window.WPSourceHelper=({
                    cache: null,
                    config: {
                        currentBlog: null,
                        currentCategory: null,
                        defaultCategories: null
                    },
                    sources: {
                        blog: null,
                        blogCategory: null
                    },
                    init: function() {
                        // Initialize sources, config, cache
                        this.sources.blog               = $(document.getElementsByClassName("wp-blog-child")[0]),
                        this.sources.blogCategory       = $("'.$this->getHtmlId().'"),
                        this.config.currentBlog         = this.sources.blog.value,
                        this.config.currentCategory     = "'.$this->getEscapedValue().'",
                        this.config.defaultCategories   = Object.clone(this.sources.blogCategory.options),
                        this.cache                      = new Object(),
                        self                            = this;

                        // Disable the categories source
                        this.sources.blogCategory.disable();

                        // Listen for blog selection
                        this.sources.blog.observe("change",function() {
                            self.config.currentBlog=this.value;

                            self.fetchCategories(
                                this,
                                function(response) {
                                    self.populateSource(self.sources.blogCategory,response);
                                }
                            );
                        });

                        // Persist config category data
                        this.sources.blogCategory.observe("change",function() {
                            self.config.currentCategory=this.value;
                        });

                        // Auto-load categories if a blog is already set
                        if(this.config.currentBlog!="-1") {
                            this.fetchCategories(
                                this.sources.blog,
                                function(response) {
                                    self.populateSource(self.sources.blogCategory,response);
                                }
                            );
                        }

                        return this;
                    },
                    fetchCategories: function(source,callback) {
                        if(typeof source!="object") {
                            alert("Cannot fetch the blog categories: no source specified.");
                            return false;
                        }

                        // Bail if `None` was selected
                        if(this.config.currentBlog=="-1") {
                            this.resetSource(this.sources.blogCategory);
                            this.sources.blogCategory.disable();
                            return false;
                        }

                        // Fetch from cache if available
                        if(typeof this.cache[this.config.currentBlog]=="object") {
                            if(typeof callback=="function") {
                                callback(this.cache[this.config.currentBlog]);
                                return this;
                            }
                        }

                        // Prep the request URL
                        var url="'.Mage::getUrl('wordpress/adminhtml_source/index',array('_current'=>true)).'";
                        url=url.replace(/\/key\//,"/blog_id/"+this.config.currentBlog+"/key/");

                        // Ask the controller to fetch from the database
                        new Ajax.Request(url,{
                            onSuccess: function(transport) {
                                var data=(transport.responseText).evalJSON();

                                self.cache[self.config.currentBlog]=data;

                                if(typeof callback=="function")
                                    callback(data);
                            },
                            onFailure: function() {
                                alert("Failed to fetch the blog categories: bad server response.");
                            }
                        });

                        return this;
                    },
                    newOption: function(attributes) {
                        var option=document.createElement("option");
                        option.value=attributes.value;
                        option.text=attributes.label;

                        if(this.config.currentCategory==attributes.value)
                            option.selected="selected";

                        return option;
                    },
                    populateSource: function(target,data) {
                        if(typeof target!="object") {
                            alert("Cannot populate the blog categories: no target specified.");
                            return false;
                        }

                        this.resetSource(target,this.config.cachedCategories);

                        for(var i=0;i<data.length;i++)
                            target.appendChild(this.newOption(data[i]));

                        this.sources.blogCategory.enable();

                        return this;
                    },
                    resetSource: function(source) {
                        if(typeof source!="object")
                            return false;

                        // Empty the source
                        source.length=0;

                        var options=this.config.defaultCategories;

                        for(var i=0;i<options.length;i++)
                            source.appendChild(options[i]);

                        return this;
                    }
                });

                window.WPSourceHelper.init();
            </script>';
        
        return $html;
    }
}