MissingSourceFile error

I created a rails application by "rails demo" command and generated a
controller "script/generate controller Say". Then I created a file in
app/view/say/hello.html.erb. I added a method "hello" in
app/controllers/say_controller.rb. From a browser to access the link
"http://localhost:3000/say/hello, I got this error:

MissingSourceFile in SayController#index

no such file to load -- sqlite3

What's wrong with my application? Did I forget anything?

Sounds like you don't have the sqlite3-ruby gem (and you'll need the
sqlite3 library as well)