Why not have the database itself create the hierarcy as a JSON file?

[timuckun]

The code uses Activerecord::Ordered hash to iterate through the returned records and creates a nested hash. Postgres and SQLite both support returning records as a json and it's possible to write the queries so that the database constructs the nested json structure. Since most database drivers are written in C and since the database itself can perform this task much faster than ruby using less memory it would be a handy feature have this as an option.

This way you may be able to just pass that JSON on to a view/front end/api client without any ruby processing at all.