gistfile1.txt

class CreateOffers < ActiveRecord::Migration
  def self.up
    create_table :offers do |t|
      t.string :title
      t.string :description
      t.integer :seller_id  ## The user_id from the seller
      t.integer :buyer_id   ## The user_id from the buyer
      t.timestamps
    end
  end

  def self.down
    drop_table :offers
  end
end
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In SQL i would write this

seller_id integer REFERENCES users(user_id) NOT NULL
buyer_id integer REFERENCES users(user_id) DEFAULT NULL