It depends: what does it do when? What does it do when the interpreter parses that line? What does it do when you call `save` on an object? What does it do when you call `create`? What does it do when you call `create!`? Etc. etc.

Of course, you always have to know how the tools you're using actually work, but Rails does have a tendency to make it seem like you don't, and to defaults that make it perhaps too easy to create bugs caused by mistaken intuition.