Create state for object in Rails

Firstly I'm sorry for my english. Ha ha

So, I'm learning to make sites with Ruby on Rails to my liking so I started with that framework. I'm stuck for a few hours because I do not find the right method to be used for something that I try to do.

I try to do a crowfunding site to carry me. I currently successfully installed and configured ActiveAdmin and have configured to create projects. I would now like to add a condition such projects. I would like 4 states: "Empty" when the project is created, but not all feature filled. "Upcoming" when it met all characteristics. And then when it is ready "Upcoming" I want my project management panel on activeadmin I have a select box that would move the project status "Validate" or "No validate".

So I thought to simply use the tool if condition to do, but my problem is I have no idea of ​​where I should put it. If I have to put in the active admin controller, a controller created for projects or in my project model. Should I create a method and if so when to call it. Maybe do something else. I struggle to see clearly in total.

Thank you in advance for your suggestions!