I create 2 select list box in my index file ,brand and model now when
user choose samsung from brand I want to list samsung phones in model
should I create a def in controller or some thing else
and if yes
what kind of code should I write
I guess that you are a beginner with Rails and suggest that you work
right through a good tutorial such as railstutorial.org, which is free
to use online. That will show you the basics of rails. Also look at
the Rails Guides.
Then you will be able to answer basic questions for yourself.