Using Sinatra to Parse JSON data from url

Here's a simple guide. I've done the same task recently.

Let's use this JSON (put it in a file called 'simple.json'):

{
    "name": "obscurite",
    "favorites": {
        "icecream": [
            "chocolate",
            "pistachio"
            ],
        "cars": [
            "ferrari",
            "porsche",
            "lamborghini"
        ]
    },
    "location": "NYC",
    "age": 100}

Parse the data of a JSON object from a url.

Step 1 is to add support for JSON parsing:

    require 'json'

Step 2 is to load in the JSON data from our new .json file:

    json_file = File.read('simple.json')
    json_data = JSON.parse(json_file)

Single out one of attributes of the json data and store it as a variable

Our data is in the form of a Hash on the outside (curly braces with key:values). Some of the values are also hashes ('favorites' and 'cars'). The values of those inner hashes are lists (Arrays in Ruby). So what we have is a hash of hashes, where some hashes are arrays.

Let's pull out my location:

    puts json_data['location'] # NYC

That was easy. It was just a top level key/value. Let's go deeper and pull out my favorite icecream(s):

    puts json_data['favorites']['icecream'] # chocolate pistachio

Now only my second favorite car:

    puts json_data['favorites']['cars'][1] # porsche

Run some arithmetic on the variable

Step 3. Let's get my age and cut it down by 50 years. Being 100 is tough!

    new_age = json_data['age'] / 2
    puts new_age

Return the result as a new variable

Step 4. Let's put the new age back into the json

    json_data['age'] = new_age
    puts json_data['age'] # 50

then post this to a new url as a new json object.

Step 5. Add the ability for your program to do an HTTP POST. Add this up at top:

    require 'net/http'

and then you can post anywhere you want. I found a fake web service you could use, if you just want to make sure the request got there.

# use this guy's fake web service page as a test. handy!
uri = URI.parse("http://jsonplaceholder.typicode.com/posts")
header = {'Content-Type'=> 'text/json'}
http = Net::HTTP.new(uri.host, uri.port)
request = Net::HTTP::Post.new(uri.request_uri, header)
request.body = json_data.to_json
response = http.request(request)

# Did we get something back?
puts response.body

On linux or mac you can open a localhost port and listen as a test:

    nc -4 -k -l -v localhost 1234

To POST to this port change the uri to:

    uri = URI.parse("http://localhost:1234")

Hope this helps. Let me know if you get stuck and I'll try to lend a hand. I'm not a ruby expert, but wanted to help a fellow explorer. Good luck.