Handlebars is a very popular templating engine, recently given much deserved attention due to the awesome Ember.js Framework. Handlebarer gives you ability to easily use Handlebars templates for both server and client side in your Rails project.

On the client-side your templates should be used with Rails' JST engine. On the server side, you can render your Handlebars templates as Rails views (similar to how you'd render ERB or HAML templates).

Lets assume you have a users controller app/controllers/users_controller.rb which in turn renders a list of users. We'd like to share that view between the client and the server.

class UsersController < ApplicationController

  def index
    @users = User.all
    respond_to do |format|
      format.html
    end
  end

end

To share our template between client and server, we need to place it under app/assets/javascripts for Sprockets' JST engine.

Lets create a views directory for our shared templates app/assets/javascripts/views and add our template there, following Rails' naming convensions.

The full template path should look like this: app/assets/javascripts/views/users/index.jst.hbs

The most significant differences between using standard server-side Ruby-based engines like ERB or HAML and using Handlebarer are:

• No access to server-side view helpers (such as url_for)
• No ruby-style instance variable like @users
• Template code is not Ruby and has to follow Handlebars' syntax rather than embedded Ruby syntax
• No partials (for now)

Our template code should look like this:

<ul class="users">
{{#users}}
<li>{{this.name}}</li>
{{/users}}
</ul>

Note that rendering this template server-side, will be done inside your application's layout. You can write your views layout file in ERB / HAML and the call to =yield will render your Handlebars template above.

Since Rails doesn't expect server-side templates to live under app/assets we need to add our client-side views path to Rails views lookup path.

Assuming we have an initializer app/config/initializers/handlebarer.rb we can add our client-side views directory like this:

Handlebarer.configure do |config|
  # make your client-side views directory discoverable to Rails
  config.views_path = Rails.root.join('app','assets','javascripts','views')
end

Internally, this adds a before_filter to ApplicationController::Base that prepends the provided path to ActionView::Context .

To render the same template from the client, we need to fetch our users list from the server and then call our JST template with that list.

First, lets change our controller to return a JSON formatted list of users when called from the client:

class UsersController < ApplicationController

  def index
    @users = User.all
    respond_to do |format|
      format.html
      format.json {
        render :json => @users.to_hbs
      }
    end
  end

end

Note the call to to_hbs on the @users collection. This ensures our users are properly serialized for use inside our template. See the Serialization section below for more details.

In our application.js file lets write the following:

//= require handlebars/runtime
//= require views/users/index

$.getJSON('/users', function(users){
  $('body').html(JST['views/users/index']({users:users}));
});

To help Handlebarer access Ruby and Rails variables inside the template, we need to employ some sort of JSON serializing before passing these variables to the template. On the server-side, this happens automagically before the template is rendered.

Internally, Handlebarer will try to call the to_hbs method on each instance variable that's passed to the template. Ruby's Hash, Array and Object classes have been extended to support this functionality. Arrays and Hashes will attempt to call the to_hbs method on their members when to_hbs is invoked on their instances. For other collection-like variables, the to_hbs method will only be invoked if they respond to a to_a method. This allows ActiveModel / ActiveRecord instance variables to automatically serialize their members before rendering.

Handlebarer does not assume your Rails models should be serialized by default. Instead, it expects you to enable serializing on desired models explicitly.

To enable this behaviour, consider the following example:

class User < ActiveRecord::Base

  include Handlebarer::Serialize

  hbs_serializable :name, :email, :favorites, :merge => false

end

The call to include Handlebarer::Serialize mixes Handlebarer::Serializer capabilities into our model class.

We can then tell the serializer which attributes we'd like to serialize, and how we'd like the serialization to work.

By default, calling hbs_serializable with no arguments will serialize all your model attributes. Lets look at two examples:

Consider the following code:

# define our model
class User < ActiveRecord::Base

  include Handlebarer::Serialize

  hbs_serializable

end

# access in controller
class UsersController < ApplicationController

  def index
    @users = User.all
    @users.to_hbs # => all available user attributes (users table columns) will be serialized
  end

  def active
    @users = User.where('active = 1').select('name, email')
    @users.to_hbs # => only name and email attributes are serialized
  end
end

For better control over which attributes are serialized, and when serializing model relationships, we can tell the serializer which attributes should always be serialized, and whether we'd like these attributes to be merged with the default attributes or not.

Consider the following code:

# define our models

class Favorite < ActiveRecord::Base

  include Handlebarer::Serialize

  hbs_serializable

  belongs_to :user

end

class User < ActiveRecord::Base

  include Handlebarer::Serialize

  hbs_serializable :favorites, :merge => true

  has_many :favorites
end

# access in controller
class UsersController < ApplicationController

  def active
    @users = User.where('active = 1').select('name, email')
    @users.to_hbs # => only name, email and favorites attributes are serialized
  end
end

In the above, we defined serialization for the User model to include the :favorites attribute, which is available because of the has_many relationship to the Favorite model. Additionally, we specified that serialization should merge model default attributes with the specified attributes, by setting :merge => true .

This will result in merging self.attributes and self.favorites on any instance of the User model when calling the to_hbs method on it.

To only serialize the specified attributes, call hbs_serializable with :merge => false .

Invokation format for hbs_serializable is:

hbs_serializable :attr1, :attr2, :attr3 ...., :merge => true/false

By default, hbs_serializable will operate with :merge => true and merge instnace attributes with specified attributes.

Handlebars has built in support for helper function registration, which is great for client-side rendering, but alas requires a bit of extra work for server-side rendering.

Lets assume you have a registered Handlebars helper defined in app/assets/javascripts/helpers/link.js that might look something like this:

Handlebars.registerHelper('link_to', function(context) {
  return '<a href="/users/' + context.id + '">' + context.name + '</a>';
});

On the client-side, you might add it to the asset pipeline like this:

//= require handlebars/runtime
//= require helpers/link.js

To use the same helper on the server-side, you'll need to configure Handlebarer like so:

Handlebarer.configure do |config|
  config.helpers_path = Rails.root.join('app','assets','javascripts','helpers')
end

When rendering your Handlebars template server-side, Handlebarer will look for any Javascript files in the helpers path and include them in the rendering context.

Please note that at the moment, Handlebarer only supports Javascript helper files rather than both Javascript and CoffeeScript.

Its recommended to configure Handlebarer inside a Rails initializer so that configuration is defined at boot time.

Assuming we have an initializer app/config/initializers/handlebarer.rb it should include:

Handlebarer.configure do |config|
  # tell Handlebarer where to find your Handlebars helpers
  config.helpers_path = Rails.root.join('app','assets','javascripts','helpers')
  # make your client-side views directory discoverable to Rails
  config.views_path = Rails.root.join('app','assets','javascripts','views')
end

In case your Rails asset pipeline is configured not to load the entire Rails environment when calling rake assets:precompile, you should include Handlebarer's configuration initalizer in your Rakefile.

Simply add require File.expand_path('../config/initializers/handlebarer', __FILE__) before require File.expand_path('../config/application', __FILE__) in your Rakefile, and ensure Handlebarer is properly configured when your assets are precompiled

Copyright (c) 2013 Zohar Arad zohar@zohararad.com

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