normalizing.rb

# graffiti.rb
class Graffiti < ActiveRecord::Base
  belongs_to :artist, class_name: "User"
  has_many :uploads
  has_many :comments
  has_many :upvotes, as: :upvotable
end

# upload.rb
class Upload < ApplicationRecord
  belongs_to :user
  belongs_to :graffiti
  has_many :upvotes, as: :upvotable
end

class Comment < ApplicationRecord
  belongs_to :user
  belongs_to :graffiti
  has_many :upvotes, as: :upvotable
end

# upvote.rb
class Upvote < ApplicationRecord
  belongs_to :user
  belongs_to :upvotable, polymorphic: true 
end

class User < ApplicationRecord
  has_many :upvotes
  has_many :uploads, through: :upvotes
  has_many :graffiti, through: :upvotes
  has_many :works, class_name: "Graffiti", foreign_key: "user_id"
  has_many :comments
end

# schema
create_table :upvotes do |t|
  t.integer  :user_id
  t.integer :upvotable_id
  t.string  :upvotable_type
  t.timestamps
end
 
add_index :upvotes, [:upvotable_type, :upvotable_id]