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KnockoutJS for Rails with Sweetness
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Knockout - easily use Knockout.js from the Rails app

If you have any questions please contact me @dnagir.

This provides a set of conveniences for you to use more like Backbone or Spine, but still fully leveraging KnockoutJS.


Add it to your Rails application's Gemfile:

gem 'knockout-rails'

Then bundle install.

Reference knockout from your JavaScript as you normally do with Rails 3.1 Assets Pipeline.



After you've referenced the knockout you can create your first persistent Model.

class @Page extends ko.Model
  @persistAt 'page' # This is enough to save the model RESTfully to `/pages/{id}` URL
  @fields 'id', 'name', 'whatever' # This is optional and will be inferred if not used

Too simple. This model conforms to the response of inherited_resources Gem.

Now you can create the model in your HTML. Note that we don't do a roundtrip to fetch the data as we already have it when rendering the view.

= content_for :script do
      // Create the viewModel with prefilled data = new Page(#{@page.to_json});
      ko.applyBindings(; // And bind everything

Of course you can manipulate the object as you wish: 'Updated page' # saves it to the server using PUT: /pages/123 '' # Assign an invalid value that is validated on the server
request = # returns the jQuery Deferred, so you can chain into it when necessary
request.always (xhr, status) ->
  # The response is 422 with JSON: {name: ["invalid name", "should not be blank"]}
  # And now we have the errors set automatically! # "invalid name, should not be blank"
  # even more than that, errors are already bound and shown in the HTML (see the view below)

Now let's see how we can show the validation errors on the page and bind everything together.{:data => {:bind =>'submit: save'}}
        %label.label{:for=>:page_name} Name
        %input#page_name{:type=>:text, :data=>{:bind=>'value: name'}}
        %span.inline-error{:data=>{:bind=>'visible:, text:'}}

Model Validations

If you are using the model, you can also take advantage of the client-side validation framework.

The client side validation works similarly to the server-side validation. This means there is only one place to check for errors, no matter where those are defined.

For example - returns the error message for the name field for both client and server side validations.

The piece of code below should explain client-side validation, including some of the options.

class @Page extends ko.Model
  @persistAt 'page'

  @validates: ->
    @acceptance  'agree_to_terms' # Value is truthy
    @presence    'name', 'body' # Non-empty, non-blank stringish value
    @email       'author' # Valid email, blanks allowed

    @presence      'password'
    @confirmation  'passwordConfirmation', {confirms: 'password'} # Blanks allowed

    # numericality:
    @numericality  'rating'
    @numericality  'rating', min: 1, max: 5

    # Inclusion/exclusion
    @inclusion   'subdomain', values: ["mine", "yours"]
    @exclusion   'subdomain', values: ["www", "www2"] 

    @format      'code', match: /\d+/ # Regex validation, blanks allowed
    @length      'name', min: 3, max: 10 # Stringish value should be with the range

    # Custom message
    @presence    'name', message: 'give me a name, yo!'

    # Conditional validation - access model using `this`
    @presence    'name', only: -> @persisted(), except: -> @id() > 5

    # Custom inline validation
    @custom 'name', (page) ->
      if ( || '').indexOf('funky') < 0 then "should be funky" else null

It is recommended to avoid custom inline validations and create your own validators instead (and maybe submit it as a Pull Request):

ko.Validations.validators.funky = (model, field, options) ->
  # options - is an optional set of options passed to the validator
  word = options.word || 'funky'
  if model[field]().indexOf(word) < 0 "should be #{word}" else null

so that you can use it like so:

@validates: ->
  funky 'name', word: 'yakk'

Here's how you would check whether the model is valid or not (assuming presence validation on name field):

page = new @Page name: ''
page.isValid() # false # "can't be blank" = 'Home'
page.isValid() # true # null

Every validator has its own set of options. But the following are applied to all of them (including yours):

  • only: -> truthy or falsy - only apply the validation when the condition is truthy. this points to the model so you can access it.
  • except: - is the opposite to only. Both only and except can be used, but you should make sure those are not mutually exclusive.

And at the end of this exercise, you can bind the errors using data-bind="text:" or any other technique.

Model Events

class @Page extends ko.Model
  @persistAt 'page'

  # Subscribe to 'sayHi' event
  @upon 'sayHi', (name) ->
    alert name + @name

page = name: 'Home'
page.trigger 'sayHi', 'Hi '
# will show "Hi Home"

Model Callbacks

The callbacks are just convenience wrappers over the predefined events. Some of them are:

class @Page extends ko.Model
  @persistAt 'page'

  @beforeSave ->
    @age = @birthdate - new Date()

# This would be similar to

class @Page extends ko.Model
  @persistAt 'page'

  @on 'beforeSave', ->
    @age = @birthdate - new Date()


This gem also includes useful bindings that you may find useful in your application. For example, you can use autosave binding by requiring knockout/bindings/autosave.

Or if you want to include all of the bindings available, then require knockout/bindings/all.

The list of currently available bindings:

  • autosave - automatically persists the model whenever any of its attributes change. Apply it to a form element. Examples: autosave: page, autosave: {model: page, when: page.isEnabled, unless: viewModel.doNotSave }. NOTE: It will not save when a model is not valid.
  • inplace - converts the input elements into inplace editing with 'Edit'/'Done' buttons. Apply it on input elements similarly to the value binding.
  • color - converts an element into a color picker. Apply it to a div element: color: page.fontColor. Depends on pakunok gem (specifically - its colorpicker asset).
  • onoff - Converts checkboxes into iOS on/off buttons. Example: onoff: page.isPublic. It depends on ios-chechboxes gem.
  • animate - runs the animation when dependent attributes change. Example: animate: {width: quotaUsed, height: quotaUsed(), duration: 2000}.
  • autocomplete - supports jQuery UI autocomplete. Example: autocomplete: {source: arrayOrObservableOrAnyObjectOrDeferred, select: observableToSetTheValueTo, label: 'nameOfTheFieldToDisplay'}. The source can support jQuery.Deferred meaning that you can simply return the result of a jQuery.ajax.

Please see the specs for more detailed instruction on how to use the specific binding.



  • Source hosted at GitHub
  • Report issues and feature requests to GitHub Issues
  • Ping me on Twitter @dnagir
  • Look at the file for current TODO list and other details.


Assuming you already cloned the repo in cd-d into it:

bundle install
# Now run the Ruby specs
bundle exec rspec spec/
# Now start JavaScript server for specs:
cd spec/dummy
bundle exec rails s
# go to http://localhost:3000/jasmine to see the results

Now you can go to spec/javascripts and start writing your specs and then modify stuff in lib/assets/javascripts to pass those.

Pull requests are very welcome, but please include the specs! It's extremely easy to write those!



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