Example: Use Acts-As-Taggable-On gem with Active-Admin and rails observers

admin-product.rb

ActiveAdmin.register Product do

  permit_params :name, :category_id, :description, :text, :slug, :in_slider, :tag_ids => []

  filter :category
  filter :base_tags
  filter :name
  filter :text
  filter :created_at
  filter :updated_at

  controller do

    def find_resource
      scoped_collection.friendly.find(params[:id])
    end

  end

  form do |f|
    f.inputs "Main info" do
      f.input :category
      f.input :name
      f.input :description
      f.input :text
      f.input :in_slider, :label => "Product in slider on Main Page"
      f.input :tags,  # Show all tags AND checked already selected one (by relations through :tags - input must named :tags)
        as: :select,
        multiple: :true,
        collection: ActsAsTaggableOn::Tag.select(:id, :name).all
    end
    f.actions
  end
  
end

application.rb

....
module MyApp
  class Application < Rails::Application

    config.active_record.observers = :product_observer, :tagging_observer

  end
end

models-product.rb

class Product < ActiveRecord::Base
  
  # Using friendly_id
  extend FriendlyId
  friendly_id :name, use: :slugged
  
  # Product belongs to Category. So tags have a :tagger_id and :tagger_type too, and should be saved through object of Category.
  belongs_to :category
  
  # Using acts_as_taggable_on gem
  acts_as_taggable # Alias for acts_as_taggable_on :tags
  acts_as_taggable_on :tags, :skills

  # Renamed a new slug after change :name field
  def should_generate_new_friendly_id?
    slug.blank? || name_changed?
  end
end

models-tagging_observer.rb

class TaggingObserver < ActiveRecord::Observer
  observe ActsAsTaggableOn::Tagging

  # Before save add :tagger_id using refs on the object witch 'taggable' and owner of this 'taggable'
  def before_save(tagging)
    tagging.tagger = tagging.taggable.category if (tagging.taggable.respond_to?(:category) and tagging.tagger != tagging.taggable.category)
  end
end

This is to edit the tags on the admin.

app/admin/tags.rb

ActiveAdmin.register ActsAsTaggableOn::Tag, as: 'Tag' do
  permit_params :name
end