What is mnesia:transform_table/3 used for?

mnesia:transform_table(Table, Fun, NewAttributes) rewrites every record in a table according to a supplied function, and updates the table's declared attribute list to match the new shape — the primary tool for evolving a table's schema after data already exists in it.

mnesia:transform_table(person,
    fun({person, Id, Name, Age}) ->
        {person, Id, Name, Age, undefined}   %% add an email field, default undefined
    end,
    [id, name, age, email]).

The transform function receives each existing record and must return the new-shaped record; Mnesia applies this to every row in the table as part of the operation. It's typically run once during a planned schema upgrade (often alongside a release upgrade), rather than something you'd call routinely as part of everyday application logic.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.