I wanted a really fly keyword tagging input in my app that let me do what I’m already pretty used to doing with Wordpress’s tagging: auto-complete existing tags to help me maintain consistency, but also let me make up new tags on the spot.

Select2 is nice as heck, and has a tagging functionality that does just what I’m looking for and is even prettier than what Wordpress has. The section on “Tagging Support” on the website looked like pretty much exactly what I wanted, but there were a few things to iron out: Firstly, I didn’t want to have to stick all the existing tags in the javascript or in the view. Yeah it’s cool that the asset pipeline lets us do .js.erb but it just feels wrong; and that list of all the existing tags could get pretty big, so jamming it all into an HTML attribute feels even more wrong. What I wanted was that AJAXy searching autocomplete where you start typing and it fetches a list from the server and that list narrows down as you type more letters. And on top of it all, I was doing this in Active Admin in a Rails 3 app.

image Select2 docs screenshot

select2-rails takes care of a good bit of that last bit, though it doesn’t do too much more than package it up for the asset pipeline. I had to wrestle with it quite a bit more, hacking bits and pieces together from different documentation, blogs, and StackOverflow threads, before everything would behave like I wanted, even though I didn’t think what I was after was particularly exotic. So naturally the right thing do to once I got it all working was to write it up here. I think even if you’re not using Active Admin, a lot of this will still help without too much adjustment, especially if you’re using Formtastic.

First off you’ll want gem 'select2-rails' and gem 'acts-as-taggable-on' in your Gemfile and bundle install’d. Then pull the select2 javascript into your app by putting //= require select2 in your active_admin.js – or application.js if you want to also have it available in non-admin parts of your app – and that same line in active_admin.css.scss. If some stuff still looks visually out of whack later on, try adding this at the end of active_admin.css.scss:

body.active_admin {
  @import 'select2'

So now we get into how to put this in your Active Admin form. We’ll make it an input for acts_as_taggable_on’s tag_list accessor because it does such a nice job of Doing What You Mean with very little fuss. Here’s a somewhat redacted excerpt from my app/admin/articles.rb:

form do |f|
  f.inputs do
    f.input :title
    f.input :content, as: :rich
    f.input :tag_list,
      label: "Tags",
      input_html: {
        data: {
          placeholder: "Enter tags",
          saved: f.object.tags.map{|t| {id: t.name, name: t.name}}.to_json,
          url: autocomplete_tags_path },
        class: 'tagselect'

As you can see, there’s quite a few attributes being given to the input’s HTML element, which Select2 will then hide and manipulate behind the scenes while presenting us the very cool tagging widget we love. The class could be whatever we want, but it’s what we’ll be using to find this element in the javascript we’ll get to momentarily.

The data hash gets placed on the input as data attributes. This is data we want to make available to said javascript. saved is for the article’s current tags, so that the widget can render those right away. Select2 expects to work with a JSON array of objects, but you’re probably wondering why I’m passing both an id and a name but setting both values to the tag’s name.

The thing is, since we’re using the tag_list accessor, we don’t really care about the tags’ IDs. I think that’s fine, after all, conceptually, a tag’s name is it’s identifying attribute. It would be a perfectly reasonable design for the tags database table to not have an id column at all and have name be the primary key – that would match our mental model of tags – but this is Rails where everything has to have an id. More to the point, Select2 won’t render the tags right, or at all, if they don’t have an id attribute with something in it. But when I used the tags’ actual ID there, the IDs were ending up among the array of tag names in the params coming in to the Rails app causing me to end up with extraneous tags getting created whose names were those IDs, and that was awful. There might be other ways around this.

The url data attribute is there to tell Select2 where to find the remote service to look up tags in for the auto-complete. It’s up to you whether you want to set this up in another controller, what you want to name it, and so on. In my case, just keeping it simple, I added it to Active Admin’s controller for my app/admin.articles.rb, like so:

controller do
  def autocomplete_tags
    @tags = ActsAsTaggableOn::Tag.
      where("name LIKE ?", "#{params[:q]}%").
    respond_to do |format|
      format.json { render json: @tags , :only => [:id, :name] }

and correspondingly, in config/routes.rb:

get '/admin/autocomplete_tags',
  to: 'admin/articles#autocomplete_tags',
  as: 'autocomplete_tags'

Fairly straightforward what’s going on here, we’ll be having Select2 pass in what we’ve typed so far in the q param and using a SQL LIKE query to give back tags to offer in the little auto-complete list.

And now, the javascript to fire up Select2’s tag input magic. Right now I just have this tacked on the end of active_admin.js but it’s a significant enough piece of code that I’d feel justified putting it in a separate file and //= require-ing it.

$(document).ready(function() {
    $('.tagselect').each(function() {
        var placeholder = $(this).data('placeholder');
        var url = $(this).data('url');
        var saved = $(this).data('saved');
            tags: true,
            placeholder: placeholder,
            minimumInputLength: 1,
            initSelection : function(element, callback){
                saved && callback(saved);
            ajax: {
                url: url,
                dataType: 'json',
                data:    function(term) { return { q: term }; },
                results: function(data) { return { results: data }; }
            createSearchChoice: function(term, data) {
                if ($(data).filter(function() {
                    return this.name.localeCompare(term)===0;
                }).length===0) {
                    return { id: term, name: term };
            formatResult:    function(item, page){ return item.name; },
            formatSelection: function(item, page){ return item.name; }

So at the top you can see I start with a jQuery selector of that “tagselect” class I put on in the input_html option, then grab the values off those data attributes, then call select2 on the element with a whole mess of the options it accepts. The most interesting bits:

  • tags: true is the simplest way to tell Select2 this is a tagging input without having to tell is what tags to autocomplete for up front.
  • minimumInputLength is how many letters we want the user to type before we start trying to suggest completions.
  • initSelection is used to set up the tagging input at the start, to get it to display what we brought in the saved data attribute.
  • ajax sets up the call to our autocomplete_tags action described before.
  • createSearchChoice is where we tell Select2 how to put the results of that call in the autocomplete list. The snarly-looking conditional here is just to filter out duplicates of tags we’ve already got picked out. As long as it’s not a duplicate, we whip up another id/name object just like we did when we set up the saved data attribute.
  • formatResult and formatSelection look for a text attribute if you don’t tell them otherwise so I’m telling them to use name.

And that’s pretty much all it takes. I had to complicate it up pretty heavily in order to see how to get it this simple, now you don’t have to. Have fun!

update 6 September 2014: Samo Zeleznik writes in:

When I create a new post with a tag that is the same as a tag that was already created prior to that it does not save it by it’s name, but by it’s id. So it creates a new entry in the tags table that has a unique id, but the name of that tag is the id of the real tag.

What I just wrote is probably a little bit confusing so let me explain it with an example: I have a post tagged with “math” and this tag has an ID of 5. Now I create a new post and I tag also tag it with “math”. Now when I save this post it will bi tagged with 5. So It creates a new tag with a unique id (6 for example) and names it 5 (id of math). Do you have any idea what could be causing this issue?

Around the same time, David Sigley tweets me with what appears to be the same issue.

As it’s been quite a while, all I could offer was that I sorta remembered having trouble with tags getting named their IDs instead of their names before and there was some hack I had to do, and I may not have done enough to point it out and explain it above. Later Samo sent me this StackOverflow question where he got it worked out, and the solution comports with the ruby code above that looks like this: f.object.tags.map{|t| {id: t.name, name: t.name}}.to_json. Note how the hash/JSON has an id key and a name key, but the value at both is the tag’s name. Later the Javascript does something siliar: return { id: term, name: term }; Then David figured it out too. I don’t have a really clear idea of why it has to be this way, it’s a hack, but there you have it.

Chuck Hoffman

Chuck Hoffman
I'm from Iowa. I sling code for a living and get pretty into it. I also do some fun things with experimental music and retro-tech.

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