company.rb

class Company < ActiveRecord::Base
  has_many :people
end

gistfile1.rb

class ExampleMigration < ActiveRecord::Migration
  def change
    create_table :companies do |t|
      t.string :name
    end
    create_table :people do |t|
      t.references :company, index: true
      t.datetime :born_on
    end
 
    add_foreign_key :people, :companies
  end
end

person.rb

class Person < ActiveRecord::Base
  belongs_to :company
end