# admin/users.rb
index do
  column("") { |user| link_to "Switch", switch_to_admin_user_path(user), method: :post, target: "_blank"}
end

member_action :switch_to, method: :post do
  user = User.find(params[:id])

  sign_out :user if user_signed_in?

  sign_in :user, user

  redirect_to root_path
end