Could I simply copy the devise app and lib (and gem?) folders into my respective project folders and have them work just as if I was using the gem? I'd like to do this so that I can really see all the code, possibly do some customization, and also avoid some "cannot find" errors in my IDE.
Is this a bad idea? If this is something that would work would I simply need to remove the gem and copy over the necessary files or would I need to "undo" things that the gem has done. Could this create problems down the road with updates?
Lastly, how exactly is devise working when I do not have all of the code in my project? It works when I'm offline so is the code just stored in some secret invisible file or something? Do some gems require an internet connection to retrieve their functionality or could any gem simply be copied and used in a project without declaring it in the gem file.
Whether this is possible, short answer: Yes. Is this a bad idea? Definitely! In fact, the code is not in your project, but when you install a gem, all its contents will be stored somewhere in your local machine. Therefore, once downloaded, you don't have to be online to use it. Well, though it's not common (and not recommended), you can clone the gem's repository to your project and set the path in the project's Gemfile.