The best way to understand how the code in Listing 7.21 works is to submit the form with some invalid signup data; the result appears in Figure 7.15, and the full debug information appears in Figure 7.16. To get a clearer picture of how Rails handles the submission, let’s take a closer look at the params hash from the debug information (Figure 7.16):

We saw starting in Section 7.1.2 that the params hash contains information about each request; in the case of a URI like /users/1, the value of params[:id] is the id of the corresponding user (1 in this example). In the case of posting to the signup form,