Happy to use an &&= operator

user_decorator.rb

class UserDecorator < Draper::Decorator
  include Draper::LazyHelpers
  delegate_all
  
  class << self
    def tagtionary
      @tagionary ||= tagtionary!.clone
    end
  
    def tagtionary!
      tag_ids = User.all.map {|u| u.tag_ids }.flatten.uniq
      tags = Tag.find(tag_ids)
      out = tags.map{|t| [t.id, t]}.to_h
      @tagionary &&= out
      out
    end
  end

  def tags
    object.tag_ids.map{|t_id| UserDecorator.tagtionary[t_id]}
  end
  
end