As a developer no one likes to write a complete documentation for a project. Butsometime we have to give some document which explain at least our architectural design of project.
If in future anyone else work on same project then he can easily understand the overview and business logic before look into the code.We should draw a simple architectural design of our project models, controller and their relationship.
There is a gem which provides all necessary diagrams - add it into development group.
$ sudo apt-get install graphviz
$ rake diagram:all
It generates all diagrams contains structural design in the /doc directory.
This gem have lot of other options.
For more details [Click here] and [click here]