Hi,

if anyone is interested in this: I didn't always want to check in the frontend if an existing tag might suit to a new blog entry, so I added a dropdown field to my blog module that gives me a list of all existing tags. The dropdown field then adds the selected tag to the comma separated tag list. It's quite straightforward:

In /blog/code/BlogEntry.php add this to the BlogEntry::getCMSFields() method before the "Tags" textfield definition:

$fields->addFieldToTab("Root.Content.Main", new DropdownField("ExistingTags", "Add existing tag/s",$this->TagsCollectionArray()), "Content");

(This field uses Javascript only and does not require a database field.)

Add the following function to the BlogEntry class to create an array with all existing tags except the ones that are already used for the blog entry:

function TagsCollectionArray() {

		// get the tags from all blog entries from the database
		$tags = DB::query("SELECT Tags FROM BlogEntry WHERE Tags != '';");

		// create a string with all tags (comma separated)	
		foreach($tags as $tag){
			$tagString .= $comma.$tag['Tags'];
			$comma = ',';
		}

		// creat an array from the comma separated string
		$allTags = explode(',',$tagString);
		
		// make sure there is no whitespace before or after the tag
		foreach($allTags as &$tag) {
			$tag = trim($tag);
		}

		// remove duplicates
		$allTags = array_unique($allTags);
		
		// get the tags from the selected blog entry
		$itemTags = split(" *, *", trim($this->Tags));

		// remove tags from the array that are already assigned to the blog entry
		$allTags = array_diff($allTags, $itemTags);
		
		// order the array alphabetically
		natcasesort($allTags);

		// add a 'select a tag' text as the first option
		array_unshift($allTags, 'select a tag');

		return $allTags;
	}

Now all that's left is to add some javascript. Still in /blog/code/BlogEntry.php add this line to the BlogEntry::getCMSFields() method to load an external Javascript file:

Requirements::javascript('blog/javascript/eventbehaviours.js');

Create the new file /blog/javascript/eventbehaviours.js:

// register an onchange behaviour
Behaviour.register({
	'#Form_EditForm_ExistingTags': {
		onchange: function() {
			Tags = document.getElementById("Form_EditForm_Tags");
			var comma = '';

			// the first option in the dropdown list is the text 'select a tag', so ignore this option
			if (this.selectedIndex > 0) {

				// add the selected tag, separated with a comma (if necessary)
				if (Tags.value != '') {
					comma = ', ';
				}
				Tags.value += comma + this.options[this.selectedIndex].innerHTML;
				
				// after the tag has been added to the blog entry remove it from the dropdown list
				this.options[this.selectedIndex] = null;
			}
		},
		initialize: function() {
			this.onchange();
		}
	}
});

That's it. Now just reload the CMS backend.

Cheers!
Anatol

Might want to add this as a recipe on the wiki to make it easy to find :D

http://doc.silverstripe.com/doku.php?id=recipes:start

Will do some time next week.

Cheers!
Anatol