This installs the gem coffee-rails as long as it’s newer than version 4.0.0 and not newer than 4.1. In other words, the >= notation always installs the latest gem, whereas the ~> 4.0.0 notation only installs updated gems representing minor point releases (e.g., from4.0.0 to 4.0.1), but not major point releases (e.g., from 4.0 to 4.1). Unfortunately, experience shows that even minor point releases can break things, so for the Ruby on Rails Tutorial we’ll err on the side of caution by including exact version numbers for all gems. You are welcome to use the most up-to-date version of any gem, including using the ~>construction in the Gemfile (which I generally recommend for more advanced users), but be warned that this may cause the tutorial to act unpredictably.
Converting the Gemfile in Listing 1.4 to use exact gem versions results in the code shown inListing 1.5. Note that we’ve also taken this opportunity to arrange for the sqlite3 gem to be included only in a development or test environment (Section 7.1.1), which prevents potential conflicts with the database used by Heroku (Section 1.5).
