ApplicationController.rb

# app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  
  # ...

  def reload_admin  
    RailsAdmin::Config.reset
    
    load(Rails.root.join('config/initializers/rails_admin.rb'))
    
    # define all models with concerns to be reloaded
    models = %w[User Group]
    models.each do |m|
      # remove model definition
      Object.send(:remove_const, m)

      # generate file_path and reload model
      model_file_path = m.to_s.underscore
      load(Rails.root.join("app/models/#{model_file_path}.rb"))
    end
  end

  # ...