api_constraint.rb

class ApiConstraint
  attr_reader :version

  def initialize(options)
    @version = options.fetch(:version)
  end

  def matches?(request)
    request
      .headers
      .fetch(:accept)
      .include?("version=#{version}")
  end
end

products_controller.rb

module Api
  module V1

    class ProductsController < ApplicationController

      # GET /products
      def index
        @products = Product.all
        render json: @products
      end

  end
end

routes.rb

require 'api_constraint'

Rails.application.routes.draw do
  namespace :api do
    scope module: :v1, constraints: ApiConstraint.new(version: 1) do

      resources :products

    end
  end
end

the controller path is app/controllers/api/v1/products_controller.rb

and the constraints path is app/constraints/api_constraint.rb

curl -H "accept: application/json; version=1" http://localhost:3000/api/products

successfully returns: {"data":[]}