The discussion related to "Listing 3.15 The Tag object models relationships to both Post and User" doesn't explain why the Tag class has a back reference to the User class. Given that back reference, it also doesn't explain why the belongsTo in Tag must use that particular form in order to support the user back reference. Consequently, while the discussion about the Post-Tag relationship is fine, the discussion of the User-Tag relationship is quite weak.
The discussion related to "Listing 3.15 The Tag object models relationships to both Post and User" doesn't explain why the Tag class has a back reference to the User class. Given that back reference, it also doesn't explain why the belongsTo in Tag must use that particular form in order to support the user back reference. Consequently, while the discussion about the Post-Tag relationship is fine, the discussion of the User-Tag relationship is quite weak.