How can I produce some simple json with this Ruby class?

Question

Here's a simple ActiveResource class. It has some instance variables and maybe even some methods, but it's not backed by any data.

ruby-1.8.7-p299 > class Box < ActiveResource::Base;
                    attr_accessor :a, :b, :c, :d;
                  end
 => nil

Let's populate it:

ruby-1.8.7-p299 > bx = Box.new; bx.a = 100; bx.b = 200;
                                bx.c = 300; bx.d = 400;
                                bx
 => #<Box:0xb5841c54 @attributes={}, @b=200, @a=100,
       @prefix_options={}, @c=300, @d=400> 

So far so good. How about we cherry-pick some of those instance variables for its JSON model? Say that we only care about b and c but not a, d, or anything else.

ruby-1.8.7-p299 > bx.to_json({:only => ['b', 'c']})
 => "{}"

That doesn't work, however, since we have no attributes called 'b' or 'c', only values. How can we wind up with something like this?

{ "box": { "b": 200, "c": 300 } }

Even better, can we get this without having to inherit from ActiveResource?

Answer 1

In an AR object, you just use the 'methods' param to to_json, like on this page: http://www.gregbenedict.com/2007/11/28/outputting-custom-model-attributes-with-to_json/ .

In a non AR object, just define a custom to_json method, where you assemble a hash of the variables you want to json-ize, then to_json it, and return it. Like, here's an (untested) example:

def to_json(options = {})
  {"box" => {"b" => b, "c" => c}}.to_json(options)
end

Answer 2

Try using YAJL-ruby to encode your hashes to json format.

require 'yajl'
hash = {:only => ['b', 'c']}
Yajl::Encoder.encode(hash)
=> "{\"only\":[\"b\",\"c\"]}"

http://rdoc.info/projects/brianmario/yajl-ruby