Rails: Best way to stub out a model with a string ID?

Question

Say I want to stub out a Product model which will have a product_id field of type string.

Do I add product_id in addition to the default id field that Rails creates?¹

rails g model product product_id:string:uniq

Or make product_id the primary key?

rails g model product product_id:primary_key

With the latter option, is there anything else to set up in addition or should it work right away?² And would storage and retrieval be as simple as:

Product.create product_id: "54c31h"
Product.find("54c31h")

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1. Note that I didn't put :index:uniq. This is because :uniq on its own creates a unique index (i.e. in the schema you would see add_index :products, :product_id, unique: true).
2. Looks like there are some extra steps:
   • https://stackoverflow.com/a/15297616/72321
   • https://stackoverflow.com/a/6714889/72321