You are absolutely correct. Using both is a very valid options.
We use ActiveRecord a lot, and then have custom SQL queries using `find_by_sql` for very complex, optimized joins. It works very well. Rails gets out of the way when we need it to.
Two caveats with find_by_sql: it’s read-only, so no insert or update commands, and it still does column-to-instance-variable monkeypatching on the object level, as opposed to the class-level monkeypatching that’s applied to normal ActiveRecord classes as soon as the DB schema is read.
For the former, there's always ActiveRecord::Base.connection.execute. For the latter, I think it's more complicated than that. There also is object-level mapping even for regular AR usage. If you do something like Foo.select("true as bar"), your Foo objects will have a bar variable available to them.
We use ActiveRecord a lot, and then have custom SQL queries using `find_by_sql` for very complex, optimized joins. It works very well. Rails gets out of the way when we need it to.