solutions

migrations.rb

class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.string :title
      t.integer :seller_id
      t.timestamps
    end
  end
end

class CreateSellers < ActiveRecord::Migration
  def change
    create_table :sellers do |t|
      t.string :name
      t.decimal :balance
      t.decimal :total_sales
      t.timestamps
    end
  end
end

class CreatePurchases < ActiveRecord::Migration
  def change
    create_table :purchases do |t|
      t.decimal :amount
      t.boolean :already_paid
      t.integer :product_id
      t.timestamps
    end
  end
end

class AddIndexes < ActiveRecord::Migration
  def change
    add_index :products, :seller_id
    add_index :purchase, :product_id
  end
end

models.rb

class Seller < ActiveRecord::Base
  has_many :products
  has_many :purchases, :through => :products
end

class Product < ActiveRecord::Base
  belongs_to :seller
  has_many :purchases
end

class Purchase < ActiveRecord::Base
  belongs_to :product
	has_one :seller, :through => :product
end