How to create many subcategories in rails?

Without using any gems how do I do this in rails? I see the gem Ancestry (https://github.com/stefankroes/ancestry)

Main Category
Sub Category
 Sub Category
    Sub-sub Category

Main Category
 Sub Category
     Sub-sub Category
          Sub-sub-sub Category


Main Category
 Sub Category
 Sub Category
 Sub Category

How would the relationship between models?

So firstly, I used ancestry. I found it saved me a ton of time and, well, pain. Tree structures can get really crazy depending on what you're using them for. If you really don't want to use a gem, I recommend creating a Category model and adding a parent_id column that can be used in a belongs_to relation to point to it. Be aware though: things like eager loading, or even querying the full tree, in this instance get very difficult, and I'm not sure how you'd even go about those things without writing a lot of raw sql. Still, if you're really motivated, here's a nifty article on how to do that as well:

https://hashrocket.com/blog/posts/recursive-sql-in-activerecord