Google Signin from server side app in ruby - ruby

Question

I have a mobile app that is signin in with google and sending a server auth code to my backend app.
I want to use this code, along with the client secrets from the google developer console, to retrieve a refresh code for retrieving data from google drive when the user is offline.
Google provides an client for auth calls in ruby, but it seems not to be maintained lately and I could not see a way to do this kind of authorisation in the docs.
In the documentation, I could find an example of how to do this on python:
from oauth2client import client
# Exchange auth code for access token, refresh token, and ID token
credentials = client.credentials_from_clientsecrets_and_code(
CLIENT_SECRET_FILE,
['https://www.googleapis.com/auth/drive.appdata', 'profile', 'email'],
auth_code)
I would like to do this in ruby through a post to their https://www.googleapis.com/oauth2/v4/token endpoint. Here is what I've tried so far:
require 'uri'
require 'net/http'
require 'json'
url = URI("https://www.googleapis.com/oauth2/v4/token")
http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true
headers = {'Content-Type': 'application/json'}
request = Net::HTTP::Post.new(url.request_uri, headers)
request.body = {
code: "#{server_auth_code_sent_to_api}",
client_id: "#{client_id_from_developer_console}",
client_secret: "#{client_secret_from_developer_console}",
grant_type: 'authorization_code',
redirect_url: '',
}.to_json
response = http.request(request)
puts JSON.parse(response.read_body)
But I keep getting the following error:
{
"error": "unsupported_grant_type",
"error_description": "Invalid grant_type: "
}
Does anybody has an idea on what I'm doing wrong, or has a working example on how to do this kind of authorisation?
Thanks in advance.

Answer 1

In case somebody stumbles here with a similar problem, what caused the request to fail was the Content-Type, and not the grant_type parameter.
Digging around in the code for the client library I saw that they use application/x-www-form-urlencoded the endpoint expects a application/x-www-form-urlencoded content type. I adjusted my code accordingly and was able to get a successful response with the valid credentials and token.
Here follows the resulting code:
require 'uri'
require 'net/http'
require 'json'
url = URI("https://www.googleapis.com/oauth2/v4/token")
params = {
"code" => "#{server_auth_code_sent_to_api}",
"client_id" => "#{client_id_from_developer_console}",
"client_secret" => "#{client_secret_from_developer_console}",
"grant_type" => "authorization_code",
"redirect_url" => "#{redirect_url_from_developer_console}",
}
response = Net::HTTP.post_form(url, params)
puts JSON.parse(response.read_body)