1 files changed, 46 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 423b5abd20..934529d496 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -224,6 +224,39 @@ module ActiveSupport
       end
     end
 
+    # Tries to find a constant with the name specified in the argument string:
+    #
+    #   "Module".safe_constantize     # => Module
+    #   "Test::Unit".safe_constantize # => Test::Unit
+    #
+    # The name is assumed to be the one of a top-level constant, no matter whether
+    # it starts with "::" or not. No lexical context is taken into account:
+    #
+    #   C = 'outside'
+    #   module M
+    #     C = 'inside'
+    #     C                    # => 'inside'
+    #     "C".safe_constantize # => 'outside', same as ::C
+    #   end
+    #
+    # nil is returned when the name is not in CamelCase or the constant (or part of it) is
+    # unknown.
+    #
+    #   "blargle".safe_constantize  # => nil
+    #   "UnknownModule".safe_constantize  # => nil
+    #   "UnknownModule::Foo::Bar".safe_constantize  # => nil
+    #
+    def safe_constantize(camel_cased_word)
+      begin
+        constantize(camel_cased_word)
+      rescue NameError => e
+        raise unless e.message =~ /uninitialized constant #{const_regexp(camel_cased_word)}$/ ||
+          e.name.to_s == camel_cased_word.to_s
+      rescue ArgumentError => e
+        raise unless e.message =~ /not missing constant #{const_regexp(camel_cased_word)}\!$/
+      end
+    end
+
     # Turns a number into an ordinal string used to denote the position in an
     # ordered sequence such as 1st, 2nd, 3rd, 4th.
     #
@@ -246,5 +279,18 @@ module ActiveSupport
         end
       end
     end
+
+    private
+
+    # Mount a regular expression that will match part by part of the constant.
+    # For instance, Foo::Bar::Baz will generate Foo(::Bar(::Baz)?)?
+    def const_regexp(camel_cased_word) #:nodoc:
+      parts = camel_cased_word.split("::")
+      last  = parts.pop
+
+      parts.reverse.inject(last) do |acc, part|
+        part.empty? ? acc : "#{part}(::#{acc})?"
+      end
+    end
   end
 end