Allow custom constraints on UUID resource routes generated by jsonapi_resources

[davidstosik]

What?

At the moment, the routing method jsonapi_resources completely overrides
options[:constraints] if the resource's resource_key_type is :uuid.
This makes it impossible to define advanced custom constraints that would apply
to those routes, be them related to the :id param or not.

How?

I'm suggesting here to set options[:constraints][:id] in the only case that
no custom constraint was specified on :id. This allows to set a custom
constraint on :id (see below why), as well as others (such as :ip).

Why?

My personal reason to request this feature is to allow myself a way to define
shortcut API endpoints such as /api/users/me, where me will stand for the
current user's UUID. This is actually already possible when ids are numeric
(because options[:constraints] is not overridden).

Example

#config/routes.rb
jsonapi_resources :users, constraints: {id: /[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}|me/}

#app/controllers/application_controller.rb
class ApplicationController < JSONAPI::ResourceController
  before_action :_load_me, only: %w(get_related_resource get_related_resources)

  def _load_me key = "user_id"
    return unless current_user && params[key] == "me"
    params[key] = current_user.id
  end
end

#app/controllers/users_controller.rb
class UsersController < ApplicationController
  before_action :_load_me, except: %w(index create)

  def _load_me
    super("id")
  end
end

[lgebhardt]

@davidstosik Thanks!