Ember on Rails: Writing data to the backend

Unfortunately, Amazon wasn't willing to buy the awesome Library Ember application we cooked up in parts 1 and 2.

I guess we'll have to hunker down, disrupt some synergies, and leverage the network effect or whatever catchphrases are used in startups these days.

In other words, it is time to add persistence to our Ember application.

Rails backend

Like in part 02, our Rails backend is going to need some work to do what we want it to.

In order for us to be able to send data to the backend and have it save it in the database, we need to add a create action to our BooksController:

class BooksController < ApplicationController
  skip_before_filter :verify_authenticity_token

  [...]

  def create
    book = Book.new(book_params)
    if book.save
      respond_with book
    else
      respond_with book, :status => 422
    end
  end

  private

  def book_params
    params.require(:book).permit(:title, :author_name)
  end
end

The create action itself is straightforward; instantiate a Book with the allowed data, attempt to save it, and respond with a JSON representation of the book.

We're skipping the authenticity token verification to make it easier to debug here and now. In a real life production app that should definitely not be skipped.

Now you should be able to create a book from the command line:

$ curl -d "book[title]=The Great Gatsby" -d "book[author_name]=F. Scott Fitzgerald" http://0.0.0.0:3000/books
{"book":{"id":6,"title":"The Great Gatsby","author_name":"F. Scott Fitzgerald"}}

Bringing it client-side

We're going to need some equivalent of the Rails create action on the Ember side of things. In Ember this is handily also called an action, but it lives inside the route to the view rather than a controller.

Create app/assets/javascripts/routes/books_new_route.js:

Library.BooksNewRoute = Ember.Route.extend({
  model: function() {
    return this.get('store').createRecord('book');
  },
  actions: {
    create: function() {
      var newBook = this.get('currentModel');
      newBook.save();
      this.transitionTo('books');
    }
  }
});

There are two things happening here (and then some):

The create action itself is fairly straightforward. It...

  1. Grabs the current model object with all the changes that has been made to it via the two input fields
  2. Saves the model to our datastore
  3. Replaces the current view with the books view (transitionTo is pretty much like redirect_to from Rails).

Note the difference from Rails here. In Rails a generic create action receives a bunch of data from the client (via params), builds a model with that data, and then persists it. Because we're on the client side of things in Ember, we can just save the model- we don't have to receive data from the form or anywhere else, the actual values of the model object have already been set.

Markup time

Time to create the actual markup we want displayed. Create a file at app/assets/javascripts/templates/books/new.handlebars (you probably need to create the directory first):

<h1>Add a book</h1>
<form {{action "create" on="submit"}}>
  <div>
    <label>
      Title<br>
      {{input type="text" value=title}}
    </label>
  </div>
  <div>
    <label>
      Author name<br>
      {{input type="text" value=authorName}}
    </label>
  </div>
  <button>Save</button>
</form>

We are using Embers {{input}} helper to generate our input fields. This allows us to pass unquoted values to each attribute, which binds that attribute to the corresponding property in our current render context. This sounds a bit complex, so let's walk through it.

value=title tells Ember to populate the HTML elements value attribute by evaluating a title property. Ember then calls title on the current render context. The current template gets its context from the model function which we created in the BooksNewRoute.

In other words, model from BooksNewRoute asks our datastore to create a new instance of a Book. That book is returned to our template, who then calls title() on it to set the value of the input element.

Data binding

In Ember terminology we have bound the value attribute to the Book models title property. Any changes we make to book.title will cause the input fields value to update - and conversely, any changes we make to the value of input field will update the title of the book.

Link it up

Now we have a template (and an implicit view) we want to be able to navigate to it. Add the following link somewhere in your application - for example in application.handlebars:

{{link-to 'Add a book' 'books.new'}}

Route to the new template

For that link to work we need to update the router at app/assets/javascripts/router.js rto map a URL to the screen that displays our "New Book" form:

Library.Router.map(function() {
  this.resource('books', function() {
    this.route('new')
  });
});

And a bit of cleanup

At some point during our work so far, Ember Rails has helpfully generated a few files in app/assets/javascripts/views/ that we don't actually need - and that are now making things a bit harder for us. So let's expose those files to a bit of ruthless refactoring:

$ rm app/assets/javascripts/views/*

Also, the original books list template we have at app/assets/javascripts/templates/books.handlebars needs to be moved inside the new books directory that holds our new template:

$ mv app/assets/javascripts/templates/books.handlebars app/assets/javascripts/templates/books/index.handlebar

Behold our UI

With all the above in place, you should be able to refresh your books list at http://0.0.0.0:3000/#/books/ and see a link to "Add a book".

Clicking that link should show a fine looking form with two input fields and a "Save" button.

Clicking "Save" creates a new Book and persists it via the API.

Victory!

Coming up

Let's recap: We now have a fully functional Ember application backend by a Rails backend. We can show data and add new data and have it persisted between sessions.

In the coming episode we'll look at how validations - in particular serverside validations - work.

If you like this tutorial, you might also be interested in my upcoming book, Ember for Rails Developers - a Rails-friendly introduction to Ember.