Add the database adapter in the generated Gemfile

railties/lib/generators/rails/app/app_generator.rb

@@ -90,6 +90,18 @@ def create_boot_file
       template "config/boot.rb"
     end
 
+    def gem_for_database
+      # %w( mysql oracle postgresql sqlite3 frontbase ibm_db )
+      case options[:database]
+      when "mysql"      then "mysql"
+      when "oracle"     then "ruby-oci8"
+      when "postgresql" then "pg"
+      when "sqlite3"    then "sqlite3-ruby"
+      when "frontbase"  then "ruby-frontbase"
+      when "ibm_db"     then "ibm_db"
+      end
+    end
+
     def create_activerecord_files
       return if options[:skip_activerecord]
       template "config/databases/#{options[:database]}.yml", "config/database.yml"

railties/lib/generators/rails/app/templates/Gemfile

@@ -13,6 +13,14 @@ gem "rails", "<%= Rails::VERSION::STRING %>"
 <%= "# " unless options.edge? %>gem "rails", :git => "git://github.com/rails/rails.git"
 <%- end -%>
 
+<% unless options[:skip_activerecord] -%>
+<% if options[:database] == 'sqlite3' -%>
+# ActiveRecord requires a database adapter. By default,
+# Rails has selected sqlite3.
+<% end -%>
+gem "<%= gem_for_database %>"
+<% end -%>
+
 ## Bundle the gems you use:
 # gem "bj"
 # gem "hpricot", "0.6"