Situations like the one illustrated above, where the same method (e.g., to_s) gets called on each element, are common enough that there’s a shorthand notation using an ampersand & and a symbol corresponding to the method:12

Using the join method (Section 4.3.1), we can create a string composed of the ids by joining them on comma-space :

We can use the above method to construct the necessary array of followed user ids by calling id on each element in user.followed_users. For example, for the first user in the database this array appears as follows:

In fact, because this sort of construction is so useful, Active Record provides it by default: