If you pick up Rails without first studying computer science or using a lower-level framework, it’s easy to reach a high level of ability without knowing a line of SQL. But then you start getting errors when your page can’t load before Heroku times out. Or you need to get into the guts of PostgreSQL to fix a weird problem. And you find that ORM, while cool, doesn’t protect you from SQL and your database entirely. This book would presume you know a fair bit about Rails and very little about SQL and key databases like PostgreSQL, and would then bridge that gap.