Build something that works
I like to use this as a reference, http://guides.rubyonrails.org/association_basics.html - it's one of the most thorough Rails-specific resources I've seen. It's pretty comprehensive and covers all the different types of relationships (e.g. belongs_to/has_many, belongs_to/has_one, etc), but it can be a little too detailed and overwhelming at times. I'd recommend reading it over a few times so you can really absorb it.