Ruby objects and JSON serialization (without Rails)

Question

I\'m trying to understand the JSON serialization landscape in Ruby. I\'m new to Ruby.

Is there any good JSON serialization options if you are not working with Rails?

Answer 1

For the JSON library to be available, you may have to install libjson-ruby from your package manager.

To use the 'json' library:

require 'json'

To convert an object to JSON (these 3 ways are equivalent):

JSON.dump object #returns a JSON string
JSON.generate object #returns a JSON string
object.to_json #returns a JSON string

To convert JSON text to an object (these 2 ways are equivalent):

JSON.load string #returns an object
JSON.parse string #returns an object

It will be a bit more difficult for objects from your own classes. For the following class, to_json will produce something like "\"#<A:0xb76e5728>\"".

class A
    def initialize a=[1,2,3], b='hello'
        @a = a
        @b = b
    end
end

This probably isn't desirable. To effectively serialise your object as JSON, you should create your own to_json method. To go with this, a from_json class method would be useful. You could extend your class like so:

class A
    def to_json
        {'a' => @a, 'b' => @b}.to_json
    end
    def self.from_json string
        data = JSON.load string
        self.new data['a'], data['b']
    end
end

You could automate this by inheriting from a 'JSONable' class:

class JSONable
    def to_json
        hash = {}
        self.instance_variables.each do |var|
            hash[var] = self.instance_variable_get var
        end
        hash.to_json
    end
    def from_json! string
        JSON.load(string).each do |var, val|
            self.instance_variable_set var, val
        end
    end
end

Then you can use object.to_json to serialise to JSON and object.from_json! string to copy the saved state that was saved as the JSON string to the object.

Answer 2

What version of Ruby are you using? ruby -v will tell you.

If it's 1.9.2, JSON is included in the standard library.

If you're on 1.8.something then do gem install json and it'll install. Then, in your code do:

require 'rubygems'
require 'json'

Then append to_json to an object and you're good to go:

asdf = {'a' => 'b'} #=> {"a"=>"b"}
asdf.to_json #=> "{"a":"b"}"

Answer 3

I used to virtus. Really powerful tool, allows to create a dynamic Ruby structure structure based on your specified classes. Easy DSL, possible to create objects from ruby hashes, there is strict mode. Check it out.

Answer 4

Actually, there is a gem called Jsonable, https://github.com/treeder/jsonable. It's pretty sweet.

Answer 5

If you're using 1.9.2 or above, you can convert hashes and arrays to nested JSON objects just using to_json.

{a: [1,2,3], b: 4}.to_json

In Rails, you can call to_json on Active Record objects. You can pass :include and :only parameters to control the output:

@user.to_json only: [:name, :email]

You can also call to_json on AR relations, like so:

User.order("id DESC").limit(10).to_json

You don't need to import anything and it all works exactly as you'd hope.