Workaround double forward error

[joshprice]

Phoenix errors when a second forward is declared to the same Plug. This can be worked around by wrapping the GraphQL.Plug.Endpoint with configuration in a new module Plug.

forward "/hello", GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.HelloWorld, :schema}
forward "/blog",  GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.SimpleBlog, :schema}

Gives this error:

== Compilation error on file web/router.ex ==
** (ArgumentError) `GraphQL.Plug.Endpoint` has already been forwarded to. A module can only be forwarded a single time.
    (phoenix) lib/phoenix/router/route.ex:171: Phoenix.Router.Route.forward_path_segments/3
    web/router.ex:29: (module)
    (stdlib) erl_eval.erl:669: :erl_eval.do_apply/6
    (elixir) lib/kernel/parallel_compiler.ex:100: anonymous fn/4 in Kernel.ParallelCompiler.spawn_compilers/8

Arguably this may be a bug in Phoenix, as this seems like a reasonable thing to do...

There are 2 workarounds:

1. The plug wrapper workaround (suggested by @chrismccord) which hides the identity of the underlying plug

   # Hello World wrapper
   defmodule HelloWorld do
     plug GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.HelloWorld, :schema}
   end
   
   # Simple Blog wrapper
   defmodule SimpleBlog do
     plug GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.SimpleBlog, :schema}
   end
   
   forward "/hello", HelloWorld
   forward "/blog",  SimpleBlog

2. This is the get/post workaround which avoids forward altogether, losing ability to handle other HTTP verbs.

   # Hello World example
   get  "/hello", GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.HelloWorld, :schema}
   post "/hello", GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.HelloWorld, :schema}
   
   # Simple Blog example
   get  "/blog",  GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.SimpleBlog, :schema}
   post "/blog",  GraphQL.Plug.Endpoint, schema: {GraphQL.Schema.SimpleBlog, :schema}

The error is fired here and perhaps could account for differing configuration?
https://github.com/phoenixframework/phoenix/blob/master/lib/phoenix/router/route.ex#L170-L172

[tillsc]

Thanks for your writeup! In my case I also needed to include use Plug.Builder in my dummy modules