Notice how we switch subjects using the subject method, replacing @user with other_user, allowing us to test the follower relationship in a natural way:

As you probably suspect, we will not be making a whole database table just to hold reverse relationships. Instead, we will exploit the underlying symmetry between followers and followed users to simulate a reverse_relationships table by passing followed_id asthe primary key.In otherwords,wherethe relationships association uses the follower_id foreign key,

the reverse_relationships association uses followed_id:  

The followers association then gets built through the reverse relationships, as shown in Listing 11.16.