how to generate migration to make references polymorphic

Question

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
  end
end

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
  end
end

You can try rails generate migration AddImageableToProducts imageable:references{polymorphic}