polymorphic_upvote_associations.rb

# graffiti.rb
class Graffiti < ActiveRecord::Base
  has_many :uploads
  has_many :upvotes, as: :upvotable
end

# upload.rb
class Upload < 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

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