gistfile1.rb

http://guides.rubyonrails.org/association_basics.html#polymorphic-associations вот твой вариант только вместо pictures будет follower

class Follower
belongs_to :followeable, polymorphic: true
belongs_to :user

class User
has_many :followers, as: :followeable

class Topic
has_many :followers, as: :followeable

class Questions
has_many :followers, as: :followeable

schema.rb
----
followers :id, followeable_id, followeable_type, user_id

users: :id

topics: :id

etc