How to write a Ruby and Rails 3 REST API

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Background

I’ve always wondered how I’d go about publishing a real REST API on the web to do something. In this example, we’ll create an employee manager app-thing. It’s not particularly interesting but it shows what “API” means. In another bit, we’ll create an “API” meaning a library to interact with this web service.

Caveats

If you are just getting started with Rails and Ruby, you might find that this tutorial is really long and includes a lot of syntax. I love Rails to death but many people say it has a “large surface area”. That means that it’s hard to learn and the API is broad and vast.

As an alternative, I suggest taking a look at Grape and Sinatra if you are finding Rails to be a little too heavy. However, make sure you read up on what features you will lose when going thinner. It’s not always clear and you might find things like autoreloading were assumed in Rails but now you have to get a plugin for Sinatra (or Grape). BTW, I think Grape by Intridea is the better Web API framework at least vs Sinatra. For pure APIs, it may be better suited for the job than stripping down Rails.

The rails app

Ok enough caveats and intro. First, create a new rails app. I’m going to assume you have RVM installed and know how to create gemsets.
rails new rest_api
cd rest_api

update:This was last tested with 3.2.12.

Database setup

In this example we are going to use Sqlite3 but you can easily substitute MySQL or some other database here. To keep this post on topic and short, we’ll use the sqlite3 default for spiking.

Generate some default UI with scaffolding.
rails g scaffold employee name:string extension:integer

Create our database tables from what the scaffolding just generated.
rake db:migrate

Ok, we’re going to pretty up the scaffold here. This is completely optional but I just hate the default.
Create app/assets/stylesheets/rest_api.css

Change app/view/layouts/application.html.erb to be:

Start rails.
rails s

Browse to http://localhost:3000/employees/. Click Create New Employee and create some records. There’s no validations or anything fancy yet so just fill in both fields accurately. Now you have some data to work with that should look something like this:

CRUD with curl

Crud is Create, Read, Update, Delete. I’ll walk through each verb with curl first.

Create
This will create a new employee using curl. Create a new file called new.xml:

curl -v -H "Content-Type: application/xml; charset=utf-8" --data-ascii @new.xml http://localhost:3000/employees.xml

Now you have a new entry in the database. You can refresh the rails /employees URL listing to watch it change.

Now let’s add xml rendering to our controller. Edit the file:
app/controllers/employees_controller.rb
Add the two format.xml lines below.

Read
Get all employees:
curl http://localhost:3000/employees.xml

Get one employee:
curl http://localhost:3000/employees/1.xml

These will just return XML to the screen. We’ll do something with this in a bit.

Update
Create update.xml:

curl -v -H "Content-Type: application/xml; charset=utf8" -T update.xml http://localhost:3000/employees/1.xml

Make sure that you have an ID of 1 in your database, otherwise you’ll get a 404 or some rails rendered error message.

Delete
I assume that you have an ID of 3 in your database. In that case, the user’s URL for the controller action show() is /3 so we can send an HTTP delete to that URL like this.

curl --request DELETE http://localhost:3000/employees/3.xml

The record will be gone from the database now if you go to the /employees page in your browser.

Ruby API Client

Now let’s make this less bound to curl. Let’s write a class called Api that represents a gem or ruby library that does some work. In our case, it’s going to make web calls (not DB calls) to update employees exactly how we were doing it with curl.

Create a new file in the rest_api/lib directory called api.rb:

This program is just like curl except we’re able to programmatically be more precise with what we’re querying and deleting. However, you’ll notice that the XML document is hardcoded in the program. So it’s not infinitely flexible. If you’re nodes are not named employees then this isn’t going to work so well. But this is just an example.

Now we’ll create a program to use api.rb. You’ll need nokogiri. So add this to your Gemfile (anywhere):

gem 'nokogiri'
And then run
bundle

This program will be a rest client that will use our api class. This api could be something you’ve published and this could be how you’d document the use of your gem to the world in your README.

Create a file named api_client.rb in the root of the rest_api rails app.

Now run with ruby api_client.rb and you should see:

Current Employees:
Fairy Faucet, Sandy Salt

Creating someone...
Current Employees:
Fairy Faucet, Sandy Salt, Bobby Flay

Updating last record ...
Current Employees:
Fairy Faucet, Sandy Salt, Robert Flaid

deleting last record ...
Current Employees:
Fairy Faucet, Sandy Salt

Depending on what dummy data you put in to begin with, the output might look different.

Rdoc

Optionally, you can create Rdoc for app. Run this rake task in the rails root:

rake doc:app

If you open doc/app/Api.html, you’ll see the Rdoc from the comments above. This is especially useful when publishing an API to the world. It’ll suck in comments from your methods, in this case the api.rb file has comments over every method definition that gets turned into pretty HTML.

Wrap up

So we have published an API over the web with Rails. It’s pretty easy because the scaffolding handles the xml requests in the respond_to blocks in the controllers. But we also wrapped this API in a utility class that we were able to run from the command line. This could easily be converted to a gem and published just as the rails app could easily be pushed to Heroku and published.

This example mimics a CRUD layer for a DB so closely that you’d never do this exactly. But hopefully it illustrates how you’d make a wrapper to a web service that you don’t have direct DB access to. For me, it was interesting to see what API really means. In terms of REST and the web, it’s simply publishing methods and data over HTTP (in this case wrapped up in XML). In terms of a library or gem, it means giving someone an object and methods to do something inside a black box you’ve designed.

20 thoughts on “How to write a Ruby and Rails 3 REST API

  1. create tutorial!

    i am new to ruby/rails (come from a c/c++/corba, java/rmi/jee/spring background).

    everything was easy to understand and everything ran as define with the exception of the client.

    i had to add the require ‘rubygems’ before it would run (see below). however, i am using snow leapord with ruby 1.8.7 and gem 1.3.7

    require ‘./lib/api.rb’
    require ‘rubygems’
    require ‘nokogiri’

    again great tutorial; just added in case someone else had the same problem.

  2. Pingback: SQUARISM ¬Ľuntil lambs become lions How to write a Ruby and Rails 3 REST API « Chandara

  3. Hi.
    Thank you so much. This example helps a lot.
    A question: i could send the xml file to web service built in .net, for example?

  4. @Ronaldo, Yes you can use webservices to bridge languages. .NET can consume a webservice as a REST client and Rails can serve the REST service.

  5. “Now run with ruby api_client.rb and you should see:” I’ve obviously missed something. If I run read from terminal in /rest_api, it just looks at me. What am I supposed to run?

  6. @Chris: See the step “Create a file named api_client.rb in the root of the rest_api rails app”. Then open a Terminal (shell) or command prompt (Windows), run the script. IE: 1) $ cd /where_you_put_your_rails_app 2) $ ruby api_client.rb . If the script doesn’t run, make sure ruby is in your path. If you have other problems, just respond.

  7. @Dillon, Thanks, I was brain dead, and not typing “ruby” before the command. Create works, but the other 3 – no luck. I went back to the curl …/employees.xml, I get nothing. If I use the curl command omitting .xml, I get a response with the html. Ruby 1.8.7 (I did add the require above), gem 1.8.11 and Rails 3.1.1

    Thanks for your help. This has been a great boost in my learning RoR!

  8. @Dillon, the server console gave me a clue with “Processing by EmployeesController#show as XML”. In the employees_controller.rb, I added the line “format.xml { render :xml => @employee }” in the show def. Interesting… I can curl with .json extension and I get the format that’s been requested for the project I’m working on.

  9. Thanks for this tutorial! I was completely in the dark about how XML requests would work in Rails but now I’ve got an inkling.

  10. Hi There,

    I would like to send .xml files to web service. I have been able to generate/write xml into a file and would like to use that file to be sent over HTTP.

    My curl command is something like
    curl -F “SUBMISSION=@submission.xml” -F “STUDY=@study.xml” -F “SAMPLE=@sample.xml” “https://somesite.com/submit/box”

    Is it possible using the tutorial you have explained.

    Thank you

  11. How hard would it be to turn this XML to JSON? Great Article either way!


    Akshay,

    This tutorial would do exactly what you need. It sends an xml file to the Server (host/url) for you; instead of CURL.

  12. Thanks for posting this!

    I’m sure I”m missing something easy, but I’m hung up after getting mysql2 installed. I’m in my app’s root directory and I’m using bash. When I try to create the database I get a bash error saying command not found. Any suggestions?

    $ mysql> create database rest_api
    -bash: mysql: command not found

  13. Great post! thanks!

    For some reason when I query for the “.xml” content nothing is returned
    So, for example, this cmd does not work curl http://localhost:3000/employees.xml
    but the same one w/o the “.xml” returns the html.

    Is there a special command or flag to ensure that xml is generated also?

  14. @cam – Are you running mysql from the mysql command line? or just typing mysql > into the CL on linux/windows? if so, you gotta start the mysql command line

    @moosya – ditto… any help on this matter would be excellent! I’m getting an error that says “error on line 1 at column 1: Extra content at the end of the document”… I hit ctrl+u and it’s empty source code.

  15. UPDATE — just figured out why the xml isn’t showing up after some googling!

    Maybe the OP can correct me if I’m wrong, but you’ll need to add a format to the controller file for the model you’re using. Just edit the controller file and add

    format.xml{ render xml: @model_name }

    after the format.html. There should be one there fore json that’s why you’re getting json results and html results. Try again and it should work :)

  16. great tuto
    just a question ,can it a mobile client (webapp) interact with your example it means consume this restful web service ??
    and again it’s a good tutorial which help me a lot
    thanks a lot :))

  17. Thanks a lot, Extremely concise and understandable. Clear idea about API and gem. Many many thanks for your time. keep going…………

  18. I go to see daily a few websites and blogs to read articles, but this
    blog presents quality based posts.

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