Including "type" attribute in json respond_with Rails 3.1

Question

It seems that when returning an object containing a "type" attribute as JSON from a Rails 3.1 application, the "type" attribute is not included. Assume I have the following:

A model with corresponding STI table Animal. Models Cat, Dog and Fish that inherit Animal.

When returning an Animal via JSON, I wish to include the "type" column, but this is not happening:

jQuery.ajax("http://localhost:3001/animals/1", {dataType: "json"});

yields:

responseText: "{"can_swim":false,"created_at":"2012-01-20T17:55:16Z","id":1,"name":"Fluffy","updated_at":"2012-01-20T17:55:16Z","weight":9.0}"

It seems like this is a problem with to_json:

bash-3.2$ rails runner 'p Animal.first.to_yaml'
"--- !ruby/object:Cat\nattributes:\n  id: 1\n  type: Cat\n  weight: 9.0\n  name: Fluffy\n  can_swim: false\n  created_at: 2012-01-20 17:55:16.090646000 Z\n  updated_at: 2012-01-20 17:55:16.090646000 Z\n"

bash-3.2$ rails runner 'p Animal.first.to_json'
"{\"can_swim\":false,\"created_at\":\"2012-01-20T17:55:16Z\",\"id\":1,\"name\":\"Fluffy\",\"updated_at\":\"2012-01-20T17:55:16Z\",\"weight\":9.0}"

Does anyone know the reasoning behind this behavior, and how to override it?

Answer 1

Override the as_json method. It's used by to_json in order to produce the output. You can do something like:

def as_json options={}
 {
   id: id,
   can_swim: can_swim,
   type: type
 }
end

Answer 2

For me, in Rails 2.3.12, the above does not work.

I can (in my Model of course) do something like this:

class Registration < ActiveRecord::Base

  def as_json(options={})
    super(options.merge(:include => [:type]))
  end

end

But that causes to_json to throw an error like this:

NoMethodError: undefined method `serializable_hash' for "Registration":String

I've worked around this with this, which is punches this method onto the object I want to export

class Registration < ActiveRecord::Base

  def as_json(options={})
    super(options.merge(:include => [:type]))
  end

  def type
    r  = self.attributes["type"]
    def r.serializable_hash(arg)
      self
    end
    r
  end
end

So on my app, I've put this into a mixin:

module RailsStiModel
  # The following two methods add the "type" parameter to the to_json output.
  # see also:
  # http://stackoverflow.com/questions/8945846/including-type-attribute-in-json-respond-with-rails-3-1/15293715#15293715
  def as_json(options={})
    super(options.merge(:include => [:type]))
  end

  def type
    r  = self.attributes["type"]
    def r.serializable_hash(arg)
      self
    end
    r
  end
end

And now in my model class(es), I add:

include RailsStiModel

Answer 3

Here is my solution which can keep the original feature of as_json.

def as_json(options={})
  options = options.try(:clone) || {}
  options[:methods] = Array(options[:methods]).map { |n| n.to_sym }
  options[:methods] |= [:type]

  super(options)
end

Answer 4

This is what i did. It just adds the missing type to the result set

  def as_json(options={})
    super(options.merge({:methods => :type}))
  end