Middleware to fix the problem with Sendgrid sending webhook POSTs with an incorrect application/json content type even though the content is actually a sequence of JSON structures separated by line breaks. Rails will try to parse the contents as a JSON document and fail. This middleware changes the content type to application/sendgrid-json, thus…

middleware.rb

Billing::Application.configure do
  config.middleware.insert_before "ActionDispatch::ParamsParser", SendgridWebhookMiddleware
end

sendgrid_webhook_middleware.rb

# -*- encoding : utf-8 -*-
require 'rack/utils'

class SendgridWebhookMiddleware
  def initialize(app, url)
    @app = app
    @url = url
  end

  def call(env)
    if env['PATH_INFO'] == url && env['REQUEST_METHOD'] == 'POST'
      if env['CONTENT_TYPE'] =~ /^application\/json(;|$)/
        env['CONTENT_TYPE'] = env['CONTENT_TYPE'].gsub(/json/, 'sendgrid-json')
      end
    end
    @app.call(env)
  end
end