Unable to autoload constant API Controller in Rails 4

Rails typically only capitalizes the first name of a module. In other words, Rails expects the namespace Api::V1::GrubsController, but you're defining it as API::V1::GrubsController.

I had a similar issue when working on a rails 6 application.

The issue was that I defined my controller as usual

class ProductsController < ApplicationController
  def index
end

Meanwhile, I had added versioning to be APIs which resulted in a namespace of this kind Api/V1.

Here's how I solved it:

I simply had to add the namespace to the controller class definition.

class Api::V1::ProductsController < ApplicationController
  def index
end

That's all.

I hope this helps

Your class name is

class API::V1::GrubsController < ApplicationController

whereas in your error its trying to look for Api::V1::GrubsController. Change the name in your class to Api