1 files changed, 79 insertions, 4 deletions
diff --git a/railties/guides/source/active_support_core_extensions.textile b/railties/guides/source/active_support_core_extensions.textile
index 3073c3a7a5..f8a72db703 100644
--- a/railties/guides/source/active_support_core_extensions.textile
+++ b/railties/guides/source/active_support_core_extensions.textile
@@ -323,6 +323,46 @@ TIP: Since +with_options+ forwards calls to its receiver they can be nested. Eac
 
 NOTE: Defined in +active_support/core_ext/object/with_options.rb+.
 
+h4. Modules and Classes
+
+h5. +remove_subclasses_of+
+
+The method +remove_subclasses_of+ receives an arbitrary number of class objects and removes their subclasses. It is a wrapper of +Class#remove_class+ explained with more details in "Class Removal FIX THIS LINK":FIXME.
+
+h5. +subclasses_of+
+
+The method +subclasses_of+ receives an arbitrary number of class objects and returns all their anonymous or reachable descendants as a single array:
+
+<ruby>
+class C; end
+subclasses_of(C) # => []
+
+subclasses_of(Integer) # => [Bignum, Fixnum]
+
+module M
+  class A; end
+  class B1 < A; end
+  class B2 < A; end
+end
+
+module N
+  class C < M::B1; end
+end
+
+subclasses_of(M::A) # => [N::C, M::B2, M::B1]
+</ruby>
+
+The order in which these classes are returned is unspecified. The returned collection may have duplicates:
+
+<ruby>
+subclasses_of(Numeric, Integer)
+# => [Bignum, Float, Fixnum, Integer, Date::Infinity, Rational, BigDecimal, Bignum, Fixnum]
+</ruby>
+
+See also +Class#subclasses+ in "Extensions to +Class+ FIXME THIS LINK":FIXME.
+
+NOTE: Defined in +active_support/core_ext/object/extending.rb+.
+
 h4. Instance Variables
 
 Active Support provides several methods to ease access to instance variables.
@@ -587,9 +627,19 @@ If for whatever reason an application loads the definition of a mailer class and
 
 NOTE: Defined in +active_support/core_ext/class/delegating_attributes.rb+.
 
-h4. Subclasses
+h4. Descendants
+
+h5. +descendents+
+
+The +descendents+ method returns all the descendants of its receiver as an array of class objects. This method performs no filtering so non-reachable classes are included, if any.
 
-The +subclasses+ method returns the names of all subclasses of a given class as an array of strings. That comprises not only direct subclasses, but all descendants down the hierarchy:
+See aso +Object#subclasses_of+, explained in "Extensions to All Objects FIX THIS LINK":FIXME.
+
+NOTE: Defined in +active_support/core_ext/object/extending.rb+.
+
+h5. +subclasses+
+
+The +subclasses+ method returns the names of all the anonymous or reachable descendants of its receiver as an array of strings:
 
 <ruby>
 class C; end
@@ -610,9 +660,9 @@ end
 M::A.subclasses # => ["N::C", "M::B2", "M::B1"]
 </ruby>
 
-The order in which these class names are returned is unspecified.
+WARNING: +ActiveRecord::Base+ redefines +subclasses+, it returns class objects, reachable or not, and it is protected.
 
-See also +Object#subclasses_of+ in "Extensions to All Objects FIX THIS LINK":FIXME.
+See aso +Object#subclasses_of+, explained in "Extensions to All Objects FIX THIS LINK":FIXME.
 
 NOTE: Defined in +active_support/core_ext/class/removal.rb+.
 
@@ -663,6 +713,31 @@ See also +Object#remove_subclasses_of+ in "Extensions to All Objects FIX THIS LI
 
 NOTE: Defined in +active_support/core_ext/class/removal.rb+.
 
+h4. Reachable Classes
+
+By definition a non-anonymous class is reachable if its name constantized is defined, and the corresponding constant evaluates to +self+:
+
+<ruby>
+class C; end
+C.reachable? # => true
+
+phantom = Object.send(:remove_const, :C)
+
+# The class object is orphan now but it still has a name.
+phantom.name # => "C"
+
+# Class name no longer available as a constant.
+phantom.reachable? # => nil
+
+# Let's define a class named "C" again.
+class C; end
+
+# Class name available as a constant, but different class object.
+phantom.reachable? # => false
+</ruby>
+
+NOTE: Defined in +active_support/core_ext/class/removal.rb+.
+
 h3. Extensions to +String+
 
 h4. +squish+