Building an RFC compliant JSON-Patch with Rails 3 and Backbone

0_README.markdown

I recently got PATCH mostly working with Backbone and Rails 3, here's what I needed to do.

Sticking Points:

1. [Rails] JSON Patch gem needs updated.
2. [Rails] Routing verb for patch missing - I believe this is fixed in Rails 4, haven't investigated.
3. [Rails] Need to parse the 'application/json-patch+json' content type in sort of a crazy way
4. [Rails] Building a patch and applying it to an activerecord model is kind of a pain, ideally we'd have a ActiveModel#apply_patch(json_patch)
5. [Backbone] Building a patch from a set of attributes is tricky - I've done the bare minimum using a loop over a hash to build add operations, which should probably cover a good chunk of cases for backbone. (I imagine most people aren't going to need the json-patch support for moving, copying or deleting keys generally.)

Relevant discussion in backbone project:

• jashkenas/backbone#2512

1_Gemfile

# Gemfile
# Fork because tenderlove hasn't released the latest RFC-compliant gem yet,
# and doesn't have the gemspec in the repo for direct linking.
gem 'hana', git: "git://github.com/adstage-david/hana.git"

2_config_routes.rb

# config/routes.rb
# ...
resources :goals do
  member do
    # Rails 3 doesn't appear to have the patch verb for routes, believe this is fixed in Rails 4.
    match :update, via: :patch
  end
end

#...

3_config_initializers_mime_types.rb

# config/initializers/mime_types.rb

# Add new mime types for use in respond_to blocks:
Mime::Type.register "application/json-patch+json", :json_patch
ActionDispatch::ParamsParser::DEFAULT_PARSERS[Mime::Type.lookup('application/json-patch+json')]=lambda do |body|
  {patch: JSON.parse(body)} # A JSON Patch is an array of operations, 
                            # params parser has to be a hash, so we just set "patch" variable
end

4_app_controllers_goals_controller.rb

# app/controllers/goals_controller.rb

class GoalsController < ApplicationController
  def update
    patch = Hana::Patch.new params[:patch]
    # Dumb update for now
    data = patch.apply({})
    update_goal(params[:id], data)
  end
  
  def update_goal(id, params)
    goal = Goal.find_by_id(params[:id])
    goal.update_attributes(data)
  end
end
    

5_6_app_assets_javascripts_adstage_model.js.coffee

# app/assets/javascripts/adstage_model.js.coffee

class adstage.Model extends Backbone.Model
  patch: (attrs = {})->
    patch = JSON.stringify(@buildPatch(attrs))
    # patch makes sure only attrs is sent, rather than full json
    # contentType sets json-patch.
    @save(attrs, {data: patch, patch: true, contentType: 'application/json-patch+json'})
  

  buildPatch: (attrs = {}, main_path = "")->
    (_.flatten(_.map(attrs, (val, key)=>
      path = "#{main_path}/#{key}"
      if _.isArray(val)
        # Technically this is just a blind replace of an old array where we could
        # be appending each item in the array to the existing array.
        # Would probably want to fix this.
        {path: path, value: val, op: 'add'}
      else if _.isObject(val)
        @buildPatch(val, path)
      else
        {path: path, value: val, op: 'add'}
    )))

Need to add in check in the controller to return :unsupported_media_type.