upvote_associations.rb

# user.rb
class User < ActiveRecord::Base 
  has_many :upvotes
  has_many :graffiti, through: :upvotes
end

# graffiti.rb
class Graffiti < ActiveRecord::Base 
  has_many :upvotes
  has_many :users, through: :upvotes
end

# upvote.rb
class Upvote < ApplicationRecord
  belongs_to :user
  belongs_to :graffiti
end

# schema
create_table "upvotes", force: :cascade do |t|
  t.integer  "user_id"
  t.integer  "graffiti_id"
  t.timestamps
end