3 files changed, 28 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/base.rb b/activerecord/lib/active_record/base.rb
index 965fe3f33a..aab832c2f7 100644
--- a/activerecord/lib/active_record/base.rb
+++ b/activerecord/lib/active_record/base.rb
@@ -13,6 +13,7 @@ require 'active_support/core_ext/string/behavior'
 require 'active_support/core_ext/kernel/singleton_class'
 require 'active_support/core_ext/module/introspection'
 require 'active_support/core_ext/object/duplicable'
+require 'active_support/core_ext/class/subclasses'
 require 'arel'
 require 'active_record/errors'
 require 'active_record/log_subscriber'
diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index a448fa1f5c..850911ebe7 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -9,6 +9,19 @@ module ActiveRecord
     end
 
     module ClassMethods
+      # Determines if one of the attributes passed in is the inheritance column,
+      # and if the inheritance column is attr accessible, it initializes an
+      # instance of the given subclass instead of the base class
+      def new(*args, &block)
+        if (attrs = args.first).is_a?(Hash)
+          if subclass = subclass_from_attrs(attrs)
+            return subclass.new(*args, &block)
+          end
+        end
+        # Delegate to the original .new
+        super
+      end
+
       # True if this isn't a concrete subclass needing a STI type condition.
       def descends_from_active_record?
         if self == Base
@@ -145,6 +158,19 @@ module ActiveRecord
 
         sti_column.in(sti_names)
       end
+
+      # Detect the subclass from the inheritance column of attrs. If the inheritance column value
+      # is not self or a valid subclass, raises ActiveRecord::SubclassNotFound
+      # If this is a StrongParameters hash, and access to inheritance_column is not permitted,
+      # this will ignore the inheritance column and return nil
+      def subclass_from_attrs(attrs)
+        subclass_name = attrs.with_indifferent_access[inheritance_column]
+        return nil if subclass_name.blank? || subclass_name == self.name
+        unless subclass = subclasses.detect { |sub| sub.name == subclass_name }
+          raise ActiveRecord::SubclassNotFound.new("Invalid single-table inheritance type: #{subclass_name} is not a subclass of #{name}")
+        end
+        subclass
+      end
     end
 
     private
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 0103de4cbd..bcfcb061f2 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -179,7 +179,7 @@ module ActiveRecord
         @collection = [:has_many, :has_and_belongs_to_many].include?(macro)
       end
 
-      # Returns a new, unsaved instance of the associated class. +options+ will
+      # Returns a new, unsaved instance of the associated class. +attributes+ will
       # be passed to the class's constructor.
       def build_association(attributes, &block)
         klass.new(attributes, &block)