A probably incorrect live-coded eTag example that doesn't use the eTag middleware

student_controller.rb

class StudentController < ApplicationController
  def show
    existing_etag = request.headers["if-none-match"]
    student = Student.where(id: params[:id]).first
    if(existing_etag)
      type, id, modified = existing_etag.split("_")
      if(type == student && id == params[:id])
        if(student.updated_at == modified)
          render :not_modified and return
        end
      end
    else
      response.headers["eTag"] = "student_#{params[:id]}_#{student.updated_at}"
      render student
  end
end