Rails: how to read an external SQLite 3 file

I have a standard Rails 4 app using MongoDB as the database. I want to be able to upload a SQLite 3 file to the application, extract some information, process it and store that in the main db.

The problem is all information I find on the topic is about using SQLite as the main database, not about reading an external file. I guess I won't be able to use ActiveRecord since that would require me to link tables to models and such, but any way that would allow me to browse the data will help. I just need to know where to start.

Thanks in advance!