Short reminder how rails use rendering

Guide says that response from the server can be: render, redirect, or just head. ActionController::Base#render designedly use convention over configuration, that means if we request products#show as HTML it will render app/view/products/show.html.erb template (if there has not been a call to render something else). ActionController::Base#render method can set up template and partial to be rendered as text, json, xml, js (that is content-type) and set the response code (:status => :ok).

For example render "edit" #or :edit will change default rendering from show.html.erb to edit.html.erb (in the same controller). We we can render template from another controller, for example carts controller with command render "carts/edit". It should be explicited with parameter render template: "edit"so we know what is going on (it will render edit template not partial). Another examples of ActionController::Base#render:

render plain: "OK"
render html: "<strong>OK</strong>".html_safe 
render xml: @product # automatically calls .to_xml
render json: @product # no need for .to_json
render js: "alert();" # will set content-type MIME text/javascript

Four options for render are:

  1. :content_type => "text/html" (or “application/json” or “application/xml” or “application/rss”) this sets MIME content-type
  2. :layout => 'special_layout' (or false) can be set to whole controller. Convention is to look first for the same name as controller
  3. :location => photo_url(@photo) sets HTTP location header
  4. :status => :ok is 200, :unprocessable_entity is 422

Redirect example is redirect_to :back.

Head example is head :created, location: photo_path(@photo) and is much clearer then format.js { render nothing: true }.

In Ruby on Rails there are 6 asset tag helpers:

<%= auto_discovery_link_tag(:rss, {action: "feed"}, {title: "RSS Feed"}) %>
<%= javascript_include_tag "main", "/photos/columns" %>
<%= stylesheet_link_tag "main", "photos/columns" %>
<%= image_tag "icons/delete.gif" %>
<%= video_tag "movie.ogg" %>
<%= audio_tag "music/first_song.mp3" %>

Asset pipeline

All asset files should be inside of app/assets, lib/assets or vendor/assets and they are served with sprockets hereof three features: fingerprint, minification and precompilation of sass and coffeescript. That features are not used in development mode, but you can see it when you run in production environment.

Rails Asset Pipeline hides (ignores) the first subfolder, for example app/assets/javascrips/posts.js will be overwritten by app/assets/custom/posts.js and served as assets/posts.js in development or included in application-123.js in production. It is important to remember that is another-folder/application.js uses require main.js that main.js file could be picked from wrong location (all asset paths are searched. If you want to add some path

# config/initializers/assets.rb
Rails.application.config.assets.paths << Rails.root.join('app', 'assets', 'adminlte', 'images')

All files should be referenced to assets pipeline by //= require posts or //= require tree . (if it was not referenced, it could be accessible, but only in development mode). If we want to include whole library (with special index file for example lib/assets/library_name/index.js) we require just folder name //= require library_name.

If we want to use controller specific assets (that is loaded only when that controller responds) we should not use *= require tree . in app/assets/stylesheets/application.css or app/assets/javascripts/application.js . Since we are including another js or css asset (that is not included in application.js/css) we have to add it to assets pipeline, for example in config/initializers/assets.rb

Rails.application.config.assets.precompile += %w( posts.js )
Rails.application.config.assets.precompile += %w( posts.css )

and include them in a view or layout file

<%= javascript_include_tag params[:controller] %>
<%= stylesheet_link_tag params[:controller] %>

Note that is posts.js is inside another folder, for example app/assets/landing/posts,js than you need to add that folder to asset paths Rails.application.config.assets.paths << Rails.root.join('app', 'assets', 'landing'). If not found, it will be ignored and not precompiled.

All non js or css files are included automatically (images, text, pdf) along with applications.js and application.css by the default matcher for compiling:

[ { |path, fn| fn =~ /app\/assets/ && !%w(.js .css).include?(File.extname(path)) }, /application.(css|js)$/ ]

If you use canonical host you need to disable it export DO_NOT_USE_CANONICAL_HOST=true and remove eventual config.action_controller.asset_host = asdasd

Compile in production mode and Heroku

RAILS_ENV=production rake db:create
RAILS_ENV=production rake assets:precompile

rails s -b -e production

One issue with assets precompile is when you use erb in coffee and + secret. If you change secrets without touching javascript files, something old from tmp cache will be used. So always change js files when you change secrets, for example echo " " >> app/assets/javascripts/ Similar dependcies could be for stylesheets scss + other asset, and you can use depend_on_asset

# config/secrets.yml
a: <%= ENV["A"] || 'a' %>

# app/assets/javascript/
a = '<%= Rails.application.secrets.a %>'

# app/assets/stylesheets/common.scss
.logo {
  background-image:url('<%= asset_path("logo.png") %> ');

rake assets:precompile
export A=b
mv logo2.png app/assets/images/logo.png
rake assets:precompile
cat public/assets/*

For Heroku you can check assets

heroku run bash
ls public/assets

in console you can see exact file name that rails believes it should serve

heroku run rails console
puts helper.asset_path("application.js")

Sometimes on heroku still need to purge 50MB tmp cache

heroku plugins:install heroku-repo
heroku repo:purge_cache


Sass is similar to scss just without brackets and with indent. I think scss is better since any css code is also scss code.

You can use asset path helpers in scss, instead of erb style background-image: url(<%= asset_url 'logo.png' %>) you can use asset-url (which replace <%=%> and url() and file name should be inside quotes). Remember that it was underscored in erb, use hyphenated in sass background-image: asset-url("logo.png"). You can not use normal url("logo.png"). Note that asset path asset-path does not work, you need to use assets url.

Note that for coffeescript you need to add extension and use this initialization file

Rails.application.config.assets.configure do |env|
  env.context_class.class_eval do
    # include MyAppHelper
    include Rails.application.routes.url_helpers

Note that if you are using sass-rails than you should use @import "filename_without_extension"; instead of require. That way you can access global namespace and you can use variables. Do not link filename with extension since than it will use plain css import rule instead of scss inserting content.

@import knows current folder so you can write only relative path.

Erb for assets could be used only for asset_data_uri (including data directly into css). Remember that precompiling assets is done only once.

You can set dependency between assets using link link_tree and link_directory directive.


If you need to include for example simple-line-icons than you can place it inslide app/assets/simple_line_icons and you need to overwrite font path, ie instead of url('../fonts/Simple-Line-Icons.eot?v=2.4.0') use asset-url('fonts/Simple-Line-Icons.eot?v=2.4.0');

// app/assets/stylesheets/pages.scss
 *= require css/simple-line-icons
@font-face {
  font-family: 'simple-line-icons';
  src: asset-url('fonts/Simple-Line-Icons.eot?v=2.4.0');
  src: asset-url('fonts/Simple-Line-Icons.eot?v=2.4.0#iefix')
  asset-url('fonts/Simple-Line-Icons.woff2?v=2.4.0') format('woff2'),
  asset-url('fonts/Simple-Line-Icons.ttf?v=2.4.0') format('truetype'),
  asset-url('fonts/Simple-Line-Icons.woff?v=2.4.0') format('woff'), asset-url('fonts/Simple-Line-Icons.svg?v=2.4.0#simple-line-icons') format('svg');
  font-weight: normal;
  font-style: normal;


You can use less and scss in same project but you can not use variables from one to another since they are not compatible.

 *= require scss_wrapper
 *= require less_wrapper


Add npm to rails app that will install dependecies in /node_modules

npm init -y # to create package.json, -y to accept defaults
npm install jquery-ui-sortable-npm

cat >> .gitignore << HERE_DOC
cat >> config/initializers/assets.rb << HERE_DOC
Rails.application.config.assets.paths << Rails.root.join('node_modules')
Rails.application.config.assets.precompile << /\.(?:svg|eot|woff|ttf)$/
git add . && git commit -m "Adding npm"

For Heroku you need to use two build packs. Follow this commit. It works for latest node and npm version. Older version could give errors:

  • bower version ~1.2 gives me error error Path must be a string. Received ... so make sure you use latest bower.

Old approach with assets from gems still works, no worry.

heroku create myapp-with-bower
heroku addons:create heroku-postgresql:hobby-dev
heroku buildpacks:set
heroku buildpacks:add --index 1
heroku buildpacks # should return  1.nodejs  2.ruby (latest will run process)
# alternativelly, we can define then in file .buildpacks
# echo '
# ' > .buildpacks
# heroku config:add BUILDPACK_URL=
git push heroku master --set-upstream

More info on heroku deploy


You can include bootstrap (which is written in less) or bootstrap-sass (if you want modifications in scss).

bower install bootstrap --save
sed -i app/assets/stylesheets/application.css -e '/require_tree/i \
 *= require bootstrap/dist/css/bootstrap'
sed -i app/assets/javascripts/application.js -e '/require_tree/i\
//= require bootstrap/dist/js/bootstrap'

git commit -am "Adding bootstrap"

Adding css and js files is working fine. There is a problem when some image/font files are hardcoded in css files. When filename is fixed and you can not include digest sha than you need to deploy files without fingerprint.

So first solution is with help of non-stupid-digest-assets gem you can add non digest version. First your assets should be seen (precompiled) with sprockets than they will be again copied without digest.

Sprockets require concatenates after sass compilation. So it’s advices to use @import sass command instead of require. @import will work also in application.css but variable definition won’t (like $var: 1;), so we need to move css -> scss.


Here is example adding fontawesome

bower install fontawesome --save
mv app/assets/stylesheets/application.css app/assets/stylesheets/application.scss
cat >> app/assets/stylesheets/application.scss << \HERE_DOC
$fa-font-path: 'font-awesome/fonts';
@import 'font-awesome/scss/font-awesome';
# don't know if this is needed for some assets
# cat >> Gemfile << HERE_DOC
# gem "non-stupid-digest-assets"

cat >> config/initializers/assets.rb << \HERE_DOC
Rails.application.config.assets.precompile << /\.(?:svg|eot|woff|woff2|ttf)$/

NonStupidDigestAssets.whitelist += [

Another solution is to overwrite @font-face css definition after you include all those icons css files

@import "simple-line-icons";

@font-face {
  font-family: 'simple-line-icons';
  src:  asset-url('landing/fonts/simple-line-icons/Simple-Line-Icons.eot?v=2.2.2');
  src:  asset-url('landing/fonts/simple-line-icons/Simple-Line-Icons.eot?#iefix&v=2.2.2') format('embedded-opentype'),
        asset-url('landing/fonts/simple-line-icons/Simple-Line-Icons.ttf?v=2.2.2') format('truetype'),
        asset-url('landing/fonts/simple-line-icons/Simple-Line-Icons.woff2?v=2.2.2') format('woff2'),
        asset-url('landing/fonts/simple-line-icons/Simple-Line-Icons.woff?v=2.2.2') format('woff'),
        asset-url('landing/fonts/simple-line-icons/Simple-Line-Icons.svg?v=2.2.2#simple-line-icons') format('svg');
  font-weight: normal;
  font-style: normal;

You can test with:

RAILS_ENV=production rake db:setup db:migrate
RAILS_ENV=production rake assets:precompile -v
RAILS_SERVE_STATIC_FILES=true rails s -e production

You can check all current asset paths and write relative to that

rails runner "puts Rails.application.config.assets.paths"


View render method has nothing to do with controller render method. <%= render 'menu' %> will render _menu.html.erb partial. <%= render 'product/edit' %> will search for product/_edit.html. Partials can be rendered with its own layout <%= render partial: 'menu', layout: 'graybar' %>

Each partial has local variable with the same name as partial and you can pass an object into it with :object <%= render partial: "customer", object: @new_customer %> or if it is an instance of Customer model shorthand is <%= render @customer %> which will use _customer.html.erb with local object customer. I do not recomend this shorthand. It is more self-explanatory when full parameters are used.

It is prefered to use local variables when passing data to partial (instead of @instance variables). This is because partials can be used from different controllers, where some @instance variable is not set. For example, <%= render partial: 'users/customer', { customer: @customer } %>. Locals of partial should be explained in a comment block:

<%# customer partial uses locals
   - customer (required, instance of Customer)
   - contact_form (true/false, default true)
 <%# (do not use contact_form||=true since it will override contact_form=false %>
   unless defined? contact_form
     contact_form = true

In layouts you can use <body class="controller-<%= controller_name %>">. controller_name is params[:controller]. In you scss you can use: .controller-home .header ... if you have something specific for each action you can add action_name class.

content_for can be controller method with this gist


If you see error ActionView::Template::Error (Unrecognised input): than you might forget semicolon ; in your less file.