Adding table index through rails

Halvor Heuch wrote in post #949981:

could someone please tell me how I can add a Mysql table index through
my rails app. without using a migration, ie from within my app ?

Why would you even want to do that? Normally the migrations are the
only part of your code that should change your DB schema, so I suspect
that you've either got very unusual needs or a design problem.

What are you ultimately trying to achieve here?