File Upload API with Grape and Rails

The main difference between the web now and the web some years ago is the front end development. Most of the work one time was done on the server and the website that appeared to users were just a bunch of divs and tables to show data coming from the back-end. Interactivity was very limited at that time and AJAX was a myth and who had the guts to do some of it could be called a hero. People went for Flash because it was a great platform which provided a higher level of web experience, animations, async operations, media management etc. But, that’s in the past. Today, JavaScript and HTML5 have changed the game, we all know, along with the great browsers that developers have on their disposal (not all of them).

I’ve been reading about serverside management in the last couple of month and I’ve realized the majority of people are going for APIs. If you have a RESTful API, virutally everything that is connected to the internet can consume it. Web sites, web apps, mobile apps, or even space ships.Instead of writing different backends for different purposes, write one solid, generic and secure API and pimp the world.

So, for those who are going to the same directions, Ruby along with Rails or Sinatra, would be the go. I personally prefer Rails for API development because it’s easy, robust and its community is awesome. While you could create a simple API using Sinatra, I’d choose Rails this time, because it has already some good functionalities under its hood, like database management. The rest of the API related functionalities are provided by Grape. Grape is described as a “micro-framework for creating REST-like APIs in Ruby”. It’s really easy to set up and use.

Assuming you already know how to create a Rails app and mount Grape on top of it (and eventually use it), let me go to the point and tell you more about Paperclip.

Paperclip is a gem that makes the file uploading process to Rack based apps, a joke. There are many other gems for that, but I found Paperclip more flexible and easy to use. As a result, I will show you how to spin the wheel with it.

So after you have created your Rails app, mounted grape and created an API class let’s install Paperclip. In your Gemfile add

gem 'paperclip'

Install your bundle with bundle install on terminal and you should be good to go. Now let’s assume you have a model called User with a name and avatar attribute. If not, just do:

rails g model User name:string avatar:string avatar_path:string

Then, before running the migration, go to the migration file of your User table and change t.string :avatar to t.attachment :avatar. It should look like this:

# in /db/migrate/

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name
      t.attachment :avatar
      t.string :avatar_path


Now run your migration:

rake db:migrate

With this we created an API resource which will just add a user with its name and upload their avatar storing a path reference of the uploaded image in :avatar_path, which obviously, is a string. So let’s create the API class in /app/api/main/api.rb. Once you do make sure the module name is the one you are mounting in your route.rb file as:

mount Main::API => '/'


# in /app/main/api.rb
module Main
	class API < Grape::API
		version 'v1', using: :header, vendor: 'some_vendor'
		format :json

We are almost ready. All we need to do is to add the resouce of our upload service to it. So inside the class write:

resource :upload do
	post do
		# takes the :avatar value and assigns it to a variable
		avatar = params[:avatar]

		# the avatar parameter needs to be converted to a
		# hash that paperclip understands as:
		attachment = {
			:filename => avatar[:filename],
			:type => avatar[:type],
			:headers => avatar[:head],
			:tempfile => avatar[:tempfile]

		# creates a new User object
		user =

		# This is the kind of File object Grape understands so let's
		# pass the hash to it
		user.avatar =

		# easy
		user.avatar_path = attachment[:filename]

		# even easier = "dummy name"

		# and...

As you see in the commented code above, the attachment hash is needed to “hashify” the file object coming from the front-end. The pass this hash to the method which returns a proper object that Grape understands and the file is uploaded.

This step was undocumented and I had to do massive searches to put together this code and, when it worked, I kissed my wife. But we are not done yet. We need to tell our User model that one attribute has to be treated as an upload:

	`# in /app/models/user.rb
class User < ActiveRecord::Base
  	attr_accessible :name, :avatar_path, :avatar
  	has_attached_file :avatar, :path => ":rails_root/public/avatars/:filename"

has_attached_file :avatar tells Ruby that this accessor, is the one who will receive the file. :path is optional, in case you have a better place where to store your file.

Now, the moment of truth. do a rails s and let’s try to upload our file from terminal using curl:

curl -F avatar=@YOUR FILE PATH HERE

Note that the parameter name must be avatar or you’ll get a 500 error. then the file path with an @ prefix. so if your file is an image.png on your desktop you should go:

curl -F avatar=@~/Desktop/image.png`

If everything went good, you should have a smile on your face once you open your public folder. In the next post I will show you how to upload a file to the same endpoint using a form and pure AJAX. Finally feel free to feedback or seek for help in the comments.

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