followable.rb

# app/models/concerns/followable.rb

module Followable
  extend ActiveSupport::Concern

  included do
    has_many :reverse_relationships, :class_name  => 'Relationship',
                                     :as          => :following,
                                     :dependent   => :destroy
    has_many :followers,             :through     => :reverse_relationships,
                                     :source      => :follower
  end
end

following.rb

# app/models/concerns/following.rb

module Following
  extend ActiveSupport::Concern

  included do
    has_many :relationships, :foreign_key => 'follower_id',
                             :dependent   => :destroy
  end

  def follow!(followable)
    followable.followers << self
  end

  def unfollow!(followable)
    relationship = relationships.find_by(following_id:  followable.id,
                                         followed_type: followable.class.name)
    relationship.destroy!
  end

  def followed?(followable)
    relationships.find_by(following_id:  followable.id,
                          followed_type: followable.class.name)
  end
end

relationship.rb

# app/models/relationship.rb

class Relationship < ActiveRecord::Base
  belongs_to :follower,  :class_name => 'User'
  belongs_to :following, :polymorphic => true

  validates_presence_of :follower, :following
end

user.rb

# app/models/user.rb

class User < ActiveRecord::Base
  include Following
  include Followable

  has_many :followed_users, :through     => :relationships,
                            :source      => :following,
                            :source_type => 'User'
end