Modeling follow relationships

A data model for follow relationships should be able to answer two questions on behalf of a user:

• Who do I follow?
• Who follows me?

In Chapter 3, Organizing Related Data, you learned to design our table structures so that all important data access can be accomplished by querying a single partition. For this reason, we're better off considering the aforementioned questions separately and designing the right table schema for each.

Outbound follows

We'll start with the question Who do I follow?. We'll want a partition per user, with each partition containing all the other users they follow:

CREATE TABLE "user_outbound_follows" ( 
  "follower_username" text, 
  "followed_username" text, 
  PRIMARY KEY ("follower_username", "followed_username") 
);

Simple enough, but there's something unusual here: there are only two columns in the table, and they're both part of the primary key. As it turns out, this is a perfectly valid way to construct a table schema; non-key columns...