? - list of postgres commands
\h - list of SQL commands
\h select(or whatever) - documentation on a specific command

(The distinction between the two types of embedded Ruby is that <% ... %> executes the code inside, while <%= ... %> executes it and inserts the result into the template.)

visit '/static_pages/home'
expect(page).to have_content('Sample App')

Visit simulates visiting the website (provided by capybara), and capybara also gives us a page variable.

$ rails generate integration_test static_pages

By the way, previous versions of Rails used PUT in place of PATCH, and Rails 4.0 still supports this usage, but PATCH matches the intended HTTP usage better and is preferred for new applications.

$ rake db:migrate VERSION=0

$ rake db:rollback

If you want to make a line break in markdown, you have to leave two or more spaces, then hit enter.
Like this, see?

Easy.

$ rails generate controller FooBars baz quux

is rolled back with:

$ rails destroy  controller FooBars baz quux

Create a new rails app without the default unit tests directory.