Aug 02, 2006

Searching for nil in all the wrong places...

A few days ago, I was adding some migrations to a Ruby on Rails project, but when I went to initialize a newly created database, I saw this error:

This being the third or so time working in Ruby, I immediately thought that it was something I did, so I started up the automatically generated scripts/application, pasted my new code into there, and proceeded to use it. The code had a couple issues that I fixed and I was confident enough to try again.

But again, I got the same error message. I then decided to add a couple prints around the new code, and re-run it, but the error above occurred without my print outs. Not sure how to proceed, I asked for some assistance from some co-workers. The advice was great, and I learned a lot about Ruby and Rails, but it still did not resolve my problem.

Finally someone recommended adding --trace to the my command:

So that is very interesting. Note how there is nothing in the above points to anything of mine. In pondering this with one of my coworkers, I explained how one of the things that I tried during debugging it was renaming the name of the class and filename, and that struck a cord.

It turns out that the mess above was all caused by my filename. I had named my file revision_add_blah_to_myApp and named my class AddBlahToMyApp. The problem with this is that the Rails convention for class names requires me to either name my file revision_add_blah_to_my_app or name my class AddBlahToMyapp. This very subtle issue caused this most unusable error message to cause a few completely wasted hours.

I have blogged before about great and not so great exceptions and their handling. I think this also falls in that category of not-so-great. It would have been nice to know that this problem originated with an attempt to load a class that did not exist, for example. In that case, I would have looked at that issue more closer than looking for references to first (and indirect references thereof) where the object invoking that method could have been nil.

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