how to generate migration to make references polymorphic

What you are trying to do is not yet implemented in the stable version of rails so Michelle's answer is the right one for now. But this feature will be implemented in rails 4 and is already available in the edge version as follows (according to this CHANGELOG):

$ rails generate migration AddImageableToProducts imageable:references{polymorphic}

Before Rails 4 there was no built-in generator for polymorphic associations. If you are using an early version of Rails generate a blank migration and then modify it by hand according to your needs.

Update:You'll need to specify which table you're changing. According to this SO answer:

class AddImageableToProducts < ActiveRecord::Migration  def up    change_table :products do |t|      t.references :imageable, polymorphic: true    end  end  def down    change_table :products do |t|      t.remove_references :imageable, polymorphic: true    end  endend

Rails 4 added a generator for polymorphic associations (see simon-olivier answer)

You could also do the following:

class AddImageableToProducts < ActiveRecord::Migration  def change    add_reference :products, :imageable, polymorphic: true, index: true  endend