The migration helper does not add an index to :owner_id and :owner_type

[dabit]

When you use the migrations helper mentioned on README, it doesn't add an index to the table

create_table :cart_items do |t|
  t.shopping_cart_item_fields # Creates the cart items fields
end

This helper should also trigger add_index :cart_items, [:owner_id, :owner_type]

[jaywilburn]

I'm getting this error:
ActiveModel::MissingAttributeError at /order_items
can't write unknown attribute price_cents

Would this be the cause of that?

Rails 5.0.3
Ruby 2.4.1