Is there some way using my relational database setup with active record to get a group of x and find all of their y's assuming that each y belong_to an x and each x has_many y's?
It should not be done this way at all. You need to join the tables together lke this.
It you have a class User that has_many Posts then you should write Post.joins(:user).where('users.x = ?', y)
This way you should return all posts of users who have a certain attribute.