How to get a particular field from JSON in Ruby - json

Question

I need to implement a simple shell utility in Ruby which parses JSON from a file and return a particular field from it.
JSON examples to be parsed:
{"status": "fail", "messages": ["Out of capacity"]}
{"status": "success", "messages": [], "result": {"node": {"ip": "1.2.3.4", "description": "", "id": 974, "name": "VM#3"}}}
Idea is to create a CLI utility with two parameters: JSON file to read and field from JSON to extract:
./get_json_field.rb ~/tmp.XXXXXX 'result.node.ip'
./get_json_field.rb ~/tmp.XXXXXX 'messages.0'
I'm struggling how to map 2nd parameter to parsed JSON data structure in Ruby. I can write an iterator for sure, splitting string to an array using dot as separator an go through it item by item but this doesn't look like elegant solution.
Any suggestions for more elegant way?

Answer 1

There is nothing wrong with splitting string and going through parts of it:
require 'json'
data1 = JSON.load('{"status": "fail", "messages": ["Out of capacity"]}')
data2 = JSON.load('{"status": "success", "messages": [], "result": {"node": {"ip": "1.2.3.4", "description": "", "id": 974, "name": "VM#3"}}}')
def get_from_json(data, query)
query.split('.').inject(data) do |memo, key|
key = key.to_i if memo.is_a? Array
memo.fetch(key)
end
end
get_from_json(data1, 'messages.0') # => "Out of capacity"
get_from_json(data2, 'result.node.ip') # => "1.2.3.4"

Answer 2

Take a look at jq it might already do what you are looking for.
jq .messages[0]
jq .node.message.ip
See http://stedolan.github.com/jq/