How to add User ID index and user:references to an Items Class

Hello im making an online retail store.

I made a mistake when creating items class.

I did

rails generate model Items title:string price:decimal description:text

I was suppose to do rails generate model Items title:string price:decimal description:text user:references

How do i add user:references without deleting my the migration file. So i will need another migration file which adds user:references?

I will also need to add a user_id index so create the new migration for user:references can i just add the following code to the migration?

add_index :Items, [:user_id, :created_at]

Thanks for any help :)