Here the followed_user_ids method is synthesized by Active Record based on the has_many :followed_users association (Listing 11.10); the result is that we need only append _ids to the association name to get the ids corresponding to the user.followed_users collection. A string of followed user ids then appears as follows:

When inserting into an SQL string, though, you don’t need to do this; the ? interpolation takes care of it for you (and in fact eliminates some database-dependent incompatibilities). This means we can use

will involve a class method in the Micropost class (a construction mentioned briefly in Section 4.4.1). A proposed implementation along these lines appears in Listing 11.43.