Ruby objects and JSON serialization (without Rails)
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?
That seems to be where this answer goes (to Rails) How to convert a Ruby object to JSON
The json gem seems to make it look like you have to write your own to_json method. I haven't been able to get to_json to work with arrays and hashes (documentation says it works with these) Is there a reason the json gem doesn't just reflect over the object and use a default serialization strategy? Isn't this how to_yaml works (guessing here)
Solution 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.
Solution 2:
Check out Oj. There are gotchas when it comes to converting any old object to JSON, but Oj can do it.
require 'oj'
class A
def initialize a=[1,2,3], b='hello'
@a = a
@b = b
end
end
a = A.new
puts Oj::dump a, :indent => 2
This outputs:
{
"^o":"A",
"a":[
1,
2,
3
],
"b":"hello"
}
Note that ^o
is used to designate the object's class, and is there to aid deserialization. To omit ^o
, use :compat
mode:
puts Oj::dump a, :indent => 2, :mode => :compat
Output:
{
"a":[
1,
2,
3
],
"b":"hello"
}
Solution 3:
If rendering performance is critical, you might also want to look at yajl-ruby, which is a binding to the C yajl library. The serialization API for that one looks like:
require 'yajl'
Yajl::Encoder.encode({"foo" => "bar"}) #=> "{\"foo\":\"bar\"}"
Solution 4:
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"}"
Solution 5:
Since I searched a lot myself to serialize a Ruby Object to json:
require 'json'
class User
attr_accessor :name, :age
def initialize(name, age)
@name = name
@age = age
end
def as_json(options={})
{
name: @name,
age: @age
}
end
def to_json(*options)
as_json(*options).to_json(*options)
end
end
user = User.new("Foo Bar", 42)
puts user.to_json #=> {"name":"Foo Bar","age":42}