Rails - Slack API OAuth Access - invalid_client_id - ruby-on-rails

Question

I'm building Slack integration for my Ruby on Rails application and I'm trying to get an access_token from the Slack API for my Slack App when a user clicks the Add to Slack button.
From Postman, I can successfully post the following:
https://slack.com/api/oauth.access?client_id=idgoes.here&client_secret=secretgoeshere&code=12345&pretty=1
However, within Rails I always get a response with invalid_client_id, regardless of the way I call the API. I have checked my ID is correct (a lot) and tried regenerating it, but I don't think that is the issue due to the postman success.
Within my get_oauth_access_token method I have tried the following implementations:
1.
rc = JSON.parse(HTTP.post('https://slack.com/api/oauth.access',
params: {
client_id: 'idgoes.here',
client_secret: 'secretgoeshere',
code: '12345'
}))
2.
response = Excon.post('https://slack.com/api/oauth.access',
headers: { 'Content-Type' => 'application/json; charset=utf-8' },
user: client_id, password: client_secret,
body: oauth_request_body.to_json)
Any implementation I try always ends up getting a invalid_client_id response.
I'm aware it may be something to do with environment config, but I'm not sure what would be helpful to debug, so please let me know what other information I can share. I'm running on localhost.
Update:
I just found out that many (maybe all) of the Slack APIs do not accept a JSON format body (which seems crazy seeing as they send a response in JSON.
Make sure to use x-www-form-urlencoded format body on your request or it will not work properly.
"Content-Type" => "application/x-www-form-urlencoded"

Answer 1

I use oauth2 gem to authorize. So I was able to get this to work by reading the slack documentation and using oauth2 in my controller:
class OauthController < ApplicationController
def authorize
options = {
site: 'https://slack.com/oauth/authorize'
}
client ||= OAuth2::Client.new(
'client-id',
'client-secret',
options
)
params = {
scope: 'incoming-webhook, commands',
redirect_uri: 'https://localhost:3000/oauth/callback'
}
redirect_to client.auth_code.authorize_url(params)
end
def authorize_callback
puts params["code"]
redirect_to root_url
end
end
Routes file:
get '/authorize', to: 'oauth#authorize'
get '/oauth/callback', to: 'oauth#authorize_callback'
Don't forget to set your callback url at Oauth settings on api.slack.com, I used localhost for testing purposes as you can see.