How to organize Rails models that are too fat?

I wouldn't do this for a few reasons.

First you violate the assumption that things will be where they should be which is probably the biggest bonus to rails in the first place. A new person can walk onto your project and navigate it quite easily if you stick model stuff in your model. If you pull it out you just add a delay and some confusion particularly if the only logic for removing something to a module is to reduce the model size.

Second you gain almost nothing from it and you lose something. File size doesn't matter these days when almost all editors and IDEs ease the navigation pain of large files. Moving stuff to a module actually takes some of this modern ease away and will require you and your colleagues or future maintainers to jump around several more files while working on one model.

That said I suspect what the hardcore rails best practice posse will tell you is that if your model is that large and complex then your design is flawed and your model is probably representing several things that could be made into separate models rather than modules.

Well, I wouldn't say that any of you are wrong to put everything in one model, but I think it is also quite valid to be able to separate various concerns. It is a trade off in the very least.

And I'm posting an answer to my own question, as I have found the Rails Way to do just that: http://github.com/jakehow/concerned_with

More information can be found here: http://m.onkey.org/2008/9/15/active-record-tips-and-tricks

I realize this is a fairly old question and it's been marked as answered, but it still has good Google juice, so I figured it was worth adding to...

Rails 3 introduced ActiveSupport::Concern, which can be used to modularize behavior that's shared across models. Or, for that matter, to slim down models that have become too fat.

DHH himself provides a nice, succinct example gist here:

https://gist.github.com/1014971