Session Problem plz help me

first i create the model on the pre-build table product ( i do not
migrate product table it is already in the database) , after that i
create the session migrate file ,
class Sessions < ActiveRecord::Migration
  def self.up
    create_table :sessions do |t|
      t.column :session_id, :string
      t.column :data, :text
      t.column :updated_at, :datetime
    end

    add_index :sessions, :session_id
    add_index :sessions, :updated_at
  end

  def self.down
    drop_table :sessions
  end
end
.
but it gives me the error
\NetBeansProjects\project1\db\migrate\002_sessions.rb:1: uninitialized
constant ActiveRecord (NameError)

can any one tell me what is the problem