How do you migrate a Mnesia record definition when its fields change?

Simply changing a -record definition in your code and recompiling doesn't retroactively update records already stored in a Mnesia table — existing rows keep whatever shape they were written with, which will mismatch the new record definition the next time your code tries to read them.

%% old: -record(person, {id, name, age}).
%% new: -record(person, {id, name, age, email}).

mnesia:transform_table(person,
    fun(P) -> P#person{email = undefined} end,
    record_info(fields, person)).

The standard fix is mnesia:transform_table/3: it walks every existing record in the table, applies a transformation function you provide to reshape each one into the new format, and updates the table's schema to match, all while the table can remain usable. This must be run once as a deliberate migration step whenever a record's field list changes, not something that happens automatically just from recompiling.

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