polymorphic を利用した has_many な関連を付与する

taggable.rb

# xxx_create_tags.rb
class CreateTags < ActiveRecord::Migration
  def change
    create_table :tags do |t|
      t.string :name, null: false
    end

    add_index :tags, :name, unique: true
  end
end

# xxx_create_taggings.rb
class CreateTaggings < ActiveRecord::Migration
  def change
    create_table :taggings do |t|
      t.references :taggable, polymorphic: true, null: false
      t.integer :tag_id, null: false
    end

    add_index :taggings, :tag_id
    add_index :taggings, [:taggable_type, :taggable_id, :tag_id], unique: true, name: 'taggings_uk'
  end
end

# tag.rb
class Tag < ActiveRecord::Base
  unloadable

  has_many :taggings, dependent: :destroy
end

# tagging.rb
class Tagging < ActiveRecord::Base
  unloadable

  belongs_to :tag
  belongs_to :taggable, polymorphic: true
end

# taggable.rb
module Taggable
  extend ActiveSupport::Concern

  included do
    has_many :taggings, as: :taggable
    has_many :tags, through: :taggings
  end

  def add_tag(name)
    return if tags.where(name: name).exists?

    tag = Tag.where(name: name).first || Tag.new(name: name)
    tags << tag
  end

  def remove_tag(name)
    tag = Tag.where(name: name).first
    tags.delete(tag) if tag && tags.include?(tag)
  end
end