[Rails] Concerns vs libraries

Concerns is a technique used to either share some behavior between
models or to break down "chubby" models into smaller, more coherent
pieces. Notice this is all about model objects.

Library code on the other hand should be self contained, independent
behaviors that can be use anywhere.
Model Concerns: Any model object where you choose to include them.
Notice that concerns "extend ActiveSupport::Concern" and therefore have
that direct dependency.

Controller Concerns: Similar to the model concerns except for
controllers.

Library: Should have no direct dependencies outside of itself (other
than libraries a library directly depends upon. i.e Rails itself has
many dependencies managed by RubyGems).

You should (in theory) be able to include a library anywhere, even in
apps that don't use Rails at all. Notice that many gems (libraries) are
useful no matter what framework you use them with. You could have a
Sinatra app and include the same gem as in a Rails app. There's no
direct dependency there.
This explains concerns pretty well I think:
http://37signals.com/svn/posts/3372-put-chubby-models-on-a-diet-with-concerns

I have read the post. actually I have it read it several times to try to
see if I could find the particular answer.

But I guess I got my question answered. Independent is the key word here
and that's what a library should be and the ability of using it anywhere. I
think I just saw a lot of bad examples of how to use libraries. I was also
getting confused since you can have concerns for models and controllers and
that is almost everything in rails. But well I think I got a pretty good
idea and will have to make design choices on my own for that case.

But I still have the feeling that this can confuse much more people since
it was introduced with one blog post and there is not big documentation
about it on rails.(Concerns I mean)

But hopefully we will get more information and new ways to use it!

Thanks for your help,

all the best,

Andre