Question

How to override default deserialization of params to model object?
In other words, how to make Rails understand camel case JSON with a snake case database?

Example: I receive params Foo object with a field fooBar and I want my Foo model to understand fooBar is in fact database field foo_bar.

"Foo": {
  "fooBar": "hello" /* fooBar is database field foo_bar */
}

class Foo < ActiveRecord::Base
  attr_accessible :foo_bar
end

class FoosController < ApplicationController
  def new
    @foo = Foo.new(params[:foo])
  end

Foo.new(params[:foo]) assumes params[:foo] contains foo_bar. Instead params[:foo] contains fooBar (in my case params contains JSON data).

I would like a clean way to handle this case, the same way a model can override as_json:

class Foo < ActiveRecord::Base
  attr_accessible :foo_bar, :another_field

  def as_json(options = nil)
    {
      fooBar: foo_bar,
      anotherField: another_field
    }
  end
end

There is a from_json method inside ActiveModel but it is not called when Foo.new(params[:foo]) is run.

I’ve read several times that overriding initialize from a model object is a terrible idea.

Answer 1

I’ve checked active_model_serializers, RABL and JBuilder. None of them allow to customize the JSON format that is received.

For that one must deal with wrap_parameters, see http://edgeapi.rubyonrails.org/classes/ActionController/ParamsWrapper.html
It works, still the code is ugly: I get JSON stuff inside my controller + the serializer/model instead of one place.

Example of use of wrap_parameters:

class EventsController < ApplicationController
  wrap_parameters :event, include: [:title, :start, :end, :allDay, :description, :location, :color]

  def create
    respond_with Event.create(params[:event])
  end
end

and then inside my model (Frederick Cheung is right on this part):

class Event < ActiveRecord::Base
  attr_accessible :title, :start, :end, :allDay, :description, :location, :color

  # JSON input allDay is all_day
  alias_attribute :allDay, :all_day

  # JSON input start is starts_at
  # +datetime+:: UNIX time
  def start=(datetime)
    self.starts_at = Time.at(datetime)
  end

  # JSON input end is starts_at
  # +datetime+:: UNIX time
  def end=(datetime)
    self.ends_at = Time.at(datetime)
  end

  # Override the JSON that is returned
  def as_json(options = nil)
    {
      id: id,
      title: title,
      start: starts_at, # ISO 8601, ex: "2011-10-28T01:22:00Z"
      end: ends_at,
      allDay: all_day,
      description: description, # Not rendered by FullCalendar
      location: location,
      color: color
    }
  end
end

For info ASP.NET MVC (with Json.NET) does it using C# decorator attributes which is pretty elegant:

class Post
{
    [JsonPropertyAttribute("title")]
    public string Title;
}

I have created a gist that shows how to implement serialization/deserialization: https://gist.github.com/3858908