Allow normal version updates within a release series

We originally chose to apply very strict versioning on the `rails` entry
in the Gemfile, because our future versioning policy was not strongly
defined.

Now it is, and our policy is very much designed on the expectation that
people will regularly update to the latest patch level in their release
series... so we should encourage that.

Of course, Gemfile.lock will do its job and prevent unplanned updates,
just as it does for every other gem in the bundle... but if you run
`bundle update`, we want to get you the latest bug/security fixes
without requiring a manual edit of the Gemfile entry.

Our current version could be a few different shapes, so it takes a bit
of work to find the right specifier, but in principle, we match anything
of the form x.y.*, where x.y matches our current release series.

4 files changed, 32 insertions, 3 deletions

diff --git a/railties/lib/rails/generators/app_base.rb b/railties/lib/rails/generators/app_base.rb
index 9036966d42..8b6295939e 100644
--- a/railties/lib/rails/generators/app_base.rb
+++ b/railties/lib/rails/generators/app_base.rb
@@ -232,11 +232,25 @@ module Rails
           ] + dev_edge_common
         else
           [GemfileEntry.version('rails',
-                            Rails::VERSION::STRING,
+                            rails_version_specifier,
                             "Bundle edge Rails instead: gem 'rails', github: 'rails/rails'")]
         end
       end
 
+      def rails_version_specifier(gem_version = Rails.gem_version)
+        if gem_version.prerelease?
+          next_series = gem_version
+          next_series = next_series.bump while next_series.segments.size > 2
+
+          [">= #{gem_version}", "< #{next_series}"]
+        elsif gem_version.segments.size == 3
+          "~> #{gem_version}"
+        else
+          patch = gem_version.segments[0, 3].join(".")
+          ["~> #{patch}", ">= #{gem_version}"]
+        end
+      end
+
       def gem_for_database
         # %w( mysql oracle postgresql sqlite3 frontbase ibm_db sqlserver jdbcmysql jdbcsqlite3 jdbcpostgresql )
         case options[:database]
diff --git a/railties/lib/rails/generators/rails/plugin/templates/%name%.gemspec b/railties/lib/rails/generators/rails/plugin/templates/%name%.gemspec
index f9465aa4af..134f573a16 100644
--- a/railties/lib/rails/generators/rails/plugin/templates/%name%.gemspec
+++ b/railties/lib/rails/generators/rails/plugin/templates/%name%.gemspec
@@ -16,7 +16,7 @@ Gem::Specification.new do |s|
 
   s.files = Dir["{app,config,db,lib}/**/*", "MIT-LICENSE", "Rakefile", "README.md"]
 
-  <%= '# ' if options.dev? || options.edge? -%>s.add_dependency "rails", "~> <%= Rails::VERSION::STRING %>"
+  <%= '# ' if options.dev? || options.edge? -%>s.add_dependency "rails", "<%= Array.wrap(rails_version_specifier).join('", "') %>"
 <% unless options[:skip_active_record] -%>
 
   s.add_development_dependency "<%= gem_for_database[0] %>"
diff --git a/railties/lib/rails/generators/rails/plugin/templates/Gemfile b/railties/lib/rails/generators/rails/plugin/templates/Gemfile
index f085a2577a..3bb4badeb4 100644
--- a/railties/lib/rails/generators/rails/plugin/templates/Gemfile
+++ b/railties/lib/rails/generators/rails/plugin/templates/Gemfile
@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 <% if options[:skip_gemspec] -%>
-<%= '# ' if options.dev? || options.edge? -%>gem 'rails', '~> <%= Rails::VERSION::STRING %>'
+<%= '# ' if options.dev? || options.edge? -%>gem 'rails', '<%= Array.wrap(rails_version_specifier).join("', '") %>'
 <% else -%>
 # Declare your gem's dependencies in <%= name %>.gemspec.
 # Bundler will treat runtime dependencies like base dependencies, and
diff --git a/railties/test/generators/generator_test.rb b/railties/test/generators/generator_test.rb
index 7871399dd7..8ef44a8dcb 100644
--- a/railties/test/generators/generator_test.rb
+++ b/railties/test/generators/generator_test.rb
@@ -80,6 +80,21 @@ module Rails
         }
         assert_equal gems.drop(2), generator.gemfile_entries
       end
+
+      def test_recommended_rails_versions
+        klass     = make_builder_class
+        generator = klass.start(['new', 'blah'])
+
+        specifier_for = -> v { generator.send(:rails_version_specifier, Gem::Version.new(v)) }
+
+        assert_equal '~> 4.1.13', specifier_for['4.1.13']
+        assert_equal ['>= 4.1.6.rc1', '< 4.2'], specifier_for['4.1.6.rc1']
+        assert_equal ['~> 4.1.7', '>= 4.1.7.1'], specifier_for['4.1.7.1']
+        assert_equal ['~> 4.1.7', '>= 4.1.7.1.2'], specifier_for['4.1.7.1.2']
+        assert_equal ['>= 4.1.7.1.rc2', '< 4.2'], specifier_for['4.1.7.1.rc2']
+        assert_equal ['>= 4.2.0.beta1', '< 4.3'], specifier_for['4.2.0.beta1']
+        assert_equal ['>= 5.0.0.beta1', '< 5.1'], specifier_for['5.0.0.beta1']
+      end
     end
   end
 end