unique validation across models

slugable.rb

module Slugable
  extend ActiveSupport::Concern

  included do
    validates :slug, uniqueness: true, presence: true, exclusion: { in: %w[admin] }

    before_validation :generate_slug
  end

  def link
    "/#{self.slug}"
  end

  private
  def generate_slug
    return true if self.slug.present?
    self.slug = self.name.to_s.gsub(/[^[:alnum:]]+/, '-').downcase
  end
end

widget.rb

class Widget < ActiveRecord::Base
  include Slugable
  validates :name, presence: true
  validate :uniqueness_of_a_slug_across_models, if: 'has_sprocket?'

    def uniqueness_of_a_slug_across_models

      errors.add(:slug, "can't be shared slug")
    end
    
  def has_sprocket!
    @has_sprocket = true
  end

  def has_sprocket?
    !!@has_sprocket
  end
  
  end 

widgets_controller.rb

class Admin::WidgetsController < ApplicationController
  before_action :set_widget, only: [:show, :edit, :update, :destroy]

  # GET /widgets
  # GET /widgets.json
  def index
    @widgets = Widget.all
  end

  # GET /widgets/1
  # GET /widgets/1.json
  def show
  end

  # GET /widgets/new
  def new
    @widget = Widget.new
  end

  # GET /widgets/1/edit
  def edit
  end

  # POST /widgets
  # POST /widgets.json
  def create
    @widget = Widget.new(widget_params)
    @widget.has_sprocket! if Sprocket.find_by(slug: params[:widget_slug])

    respond_to do |format|
      if @widget.save
        format.html { redirect_to [:admin, @widget], notice: 'widget was successfully created.' }
        format.json { render action: 'show', status: :created, location: @widget }
      else
        format.html { render action: 'new' }
        format.json { render json: @widget.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /widgets/1
  # PATCH/PUT /widgets/1.json
  def update
    respond_to do |format|
      if @widget.update(widget_params)
        format.html { redirect_to [:admin, @widget], notice: 'widget was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @widget.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /widgets/1
  # DELETE /widgets/1.json
  def destroy
    @widget.destroy
    respond_to do |format|
      format.html { redirect_to admin_widgets_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_widget
      @widget = Widget.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def widget_params
      params.require(:widget).permit(:name, :slug, :description)
    end
end