Prevent all writes

I'm am writing a Rails app as a second front-end to another system. That
system handles all writing to the database.

The database is very large, with hundreds of thousands of records. It is
not practical to duplicate the database nor try to replicate the
functionality that updates and maintains it. It is also highly dependent
on the underlying file system, all of which is my way of saying that I
must "work hot" on the live db.

My app must be read only. Is there any way to absolutely prevent all
writes from Rails to the database (either within my app, or in MySql) so
I don't accidentally or through carelessness have Rails drop a table or
overwrite large portions of data?

Configure a read only user within mysql and use that in your database.yml file.

I do this at work when I need to query our production database just to be sure I don't mess up and it works fine.

something like this in mysql:

grant select on productiondb.* to identified by 'password'

Check the docs for the exact syntax.