When it comes to building a Rails application, the one site I constantly reference when finding out attributes for methods such as create_table or finding out what CallBacks I can use to hook in additional functionality, Rails Manual has been invaluable.
Just recently I updated the site with the latest version of Rannotate, which has a ton of new features and functionality enhancement by Conor Hunt.
Most importantly is the ability for you to be able to have multiple versions of a library, such as the first major version of Rails 1.0.0 or the latest version being 1.1.2. You can see all the methods within each version, as well as add comments to each of the methods within each.
The idea is to replicate the same usefulness of the PHP documentation, which I am personally sure is one of the main reasons why PHP has become so widely adopted.
It is important to note that although Rannotate is used on RailsManual exclusively for Ruby on Rails functionality, the library is excellent for documenting all types of Ruby and Rails applications, and could very well be hosted internally for your own API.
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