diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 1a2cd15458..0ef11609ee 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,11 @@
 *SVN*
 
+* Allow :dependent options to be used with polymorphic joins. #3820 [Rick Olson]
+
+    class Foo < ActiveRecord::Base
+      has_many :attachments, :as => :attachable, :dependent => :delete_all
+    end
+
 * Upgrade to Transaction::Simple 1.3 [Jamis Buck]
 
 * Catch FixtureClassNotFound when using instantiated fixtures on a fixture that has no ActiveRecord model [Rick Olson]
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index a098c0f326..5d9672a0d9 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -485,7 +485,7 @@ module ActiveRecord
                 
                 if association.updated?
                   self["#{reflection.primary_key_name}"] = association.id
-                  self["#{reflection.options[:foreign_type]}"] = ActiveRecord::Base.send(:class_name_of_active_record_descendant, association.class).to_s
+                  self["#{reflection.options[:foreign_type]}"] = association.class.base_class.name.to_s
                 end
               end
             EOF
@@ -811,13 +811,20 @@ module ActiveRecord
 
           # See HasManyAssociation#delete_records.  Dependent associations
           # delete children, otherwise foreign key is set to NULL.
+
+          # Add polymorphic type if the :as option is present
+          dependent_conditions = %(#{reflection.primary_key_name} = \#{record.quoted_id})
+          if reflection.options[:as]
+            dependent_conditions += " AND #{reflection.options[:as]}_type = '#{base_class.name}'"
+          end
+
           case reflection.options[:dependent]
             when :destroy, true  
               module_eval "before_destroy '#{reflection.name}.each { |o| o.destroy }'"
             when :delete_all
-              module_eval "before_destroy { |record| #{reflection.class_name}.delete_all(%(#{reflection.primary_key_name} = \#{record.quoted_id})) }"
+              module_eval "before_destroy { |record| #{reflection.class_name}.delete_all(%(#{dependent_conditions})) }"
             when :nullify
-              module_eval "before_destroy { |record| #{reflection.class_name}.update_all(%(#{reflection.primary_key_name} = NULL),  %(#{reflection.primary_key_name} = \#{record.quoted_id})) }"
+              module_eval "before_destroy { |record| #{reflection.class_name}.update_all(%(#{reflection.primary_key_name} = NULL),  %(#{dependent_conditions})) }"
             when nil, false
               # pass
             else
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 32e812a86a..e54e38e184 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -118,10 +118,11 @@ module ActiveRecord
 
       def primary_key_name
         return @primary_key_name if @primary_key_name
-        
-        case macro
-          when :belongs_to
+        case
+          when macro == :belongs_to
             @primary_key_name = options[:foreign_key] || class_name.foreign_key
+          when options[:as]
+            @primary_key_name = options[:foreign_key] || "#{options[:as]}_id"
           else
             @primary_key_name = options[:foreign_key] || active_record.name.foreign_key
         end
diff --git a/activerecord/test/associations_join_model_test.rb b/activerecord/test/associations_join_model_test.rb
index b14693d7a6..2d74895664 100644
--- a/activerecord/test/associations_join_model_test.rb
+++ b/activerecord/test/associations_join_model_test.rb
@@ -69,13 +69,72 @@ class AssociationsJoinModelTest < Test::Unit::TestCase
   end
 
   def test_create_polymorphic_has_many_with_scope
-    tagging = posts(:welcome).taggings.create(:tag => tags(:general))
+    old_count = posts(:welcome).taggings.count
+    tagging = posts(:welcome).taggings.create(:tag => tags(:misc))
     assert_equal "Post", tagging.taggable_type
+    assert_equal old_count+1, posts(:welcome).taggings.count
   end
 
   def test_create_polymorphic_has_one_with_scope
-    tagging = posts(:welcome).tagging.create(:tag => tags(:general))
+    old_count = Tagging.count
+    tagging = posts(:welcome).tagging.create(:tag => tags(:misc))
     assert_equal "Post", tagging.taggable_type
+    assert_equal old_count+1, Tagging.count
+  end
+
+  def test_delete_polymorphic_has_many_with_delete_all
+    assert_equal 1, posts(:welcome).taggings.count
+    posts(:welcome).taggings.first.update_attribute :taggable_type, 'PostWithHasManyDeleteAll'
+    post = find_post_with_dependency(1, :has_many, :taggings, :delete_all)
+
+    old_count = Tagging.count
+    post.destroy
+    assert_equal old_count-1, Tagging.count
+    assert_equal 0, posts(:welcome).taggings.count
+  end
+
+  def test_delete_polymorphic_has_many_with_destroy
+    assert_equal 1, posts(:welcome).taggings.count
+    posts(:welcome).taggings.first.update_attribute :taggable_type, 'PostWithHasManyDestroy'
+    post = find_post_with_dependency(1, :has_many, :taggings, :destroy)
+
+    old_count = Tagging.count
+    post.destroy
+    assert_equal old_count-1, Tagging.count
+    assert_equal 0, posts(:welcome).taggings.count
+  end
+
+  def test_delete_polymorphic_has_many_with_nullify
+    assert_equal 1, posts(:welcome).taggings.count
+    posts(:welcome).taggings.first.update_attribute :taggable_type, 'PostWithHasManyNullify'
+    post = find_post_with_dependency(1, :has_many, :taggings, :nullify)
+
+    old_count = Tagging.count
+    post.destroy
+    assert_equal old_count, Tagging.count
+    assert_equal 0, posts(:welcome).taggings.count
+  end
+
+  def test_delete_polymorphic_has_one_with_destroy
+    assert posts(:welcome).tagging
+    posts(:welcome).tagging.update_attribute :taggable_type, 'PostWithHasOneDestroy'
+    post = find_post_with_dependency(1, :has_one, :tagging, :destroy)
+
+    old_count = Tagging.count
+    post.destroy
+    assert_equal old_count-1, Tagging.count
+    assert_nil posts(:welcome).tagging(true)
+  end
+
+  def test_delete_polymorphic_has_one_with_nullify
+    assert posts(:welcome).tagging
+    posts(:welcome).tagging.update_attribute :taggable_type, 'PostWithHasOneNullify'
+    post = find_post_with_dependency(1, :has_one, :tagging, :nullify)
+
+    old_count = Tagging.count
+    post.destroy
+    assert_equal old_count, Tagging.count
+    assert_nil posts(:welcome).tagging(true)
   end
 
   def test_has_many_with_piggyback
@@ -139,4 +198,15 @@ class AssociationsJoinModelTest < Test::Unit::TestCase
     tagging.destroy
     assert posts(:welcome, :reload)[:taggings_count].zero?
   end
+
+  private
+    # create dynamic Post models to allow different dependency options
+    def find_post_with_dependency(post_id, association, association_name, dependency)
+      class_name = "PostWith#{association.to_s.classify}#{dependency.to_s.classify}"
+      Post.find(post_id).update_attribute :type, class_name
+      klass = Object.const_set(class_name, Class.new(ActiveRecord::Base))
+      klass.set_table_name 'posts'
+      klass.send(association, association_name, :as => :taggable, :dependent => dependency)
+      klass.find(post_id)
+    end
 end
diff --git a/activerecord/test/fixtures/post.rb b/activerecord/test/fixtures/post.rb
index 7c1f8e1961..e5b95aff32 100644
--- a/activerecord/test/fixtures/post.rb
+++ b/activerecord/test/fixtures/post.rb
@@ -18,7 +18,7 @@ class Post < ActiveRecord::Base
   has_many :special_comments
 
   has_and_belongs_to_many :categories
-  has_and_belongs_to_many :special_categories, :join_table => "categories_posts"
+  has_and_belongs_to_many :special_categories, :join_table => "categories_posts", :association_foreign_key => 'category_id'
 
   has_many :taggings, :as => :taggable
   has_many :tags, :through => :taggings
diff --git a/activerecord/test/fixtures/taggings.yml b/activerecord/test/fixtures/taggings.yml
index dced625580..38ec1ba5b1 100644
--- a/activerecord/test/fixtures/taggings.yml
+++ b/activerecord/test/fixtures/taggings.yml
@@ -9,3 +9,9 @@ thinking_general:
   tag_id: 1
   taggable_id: 2
   taggable_type: Post
+
+fake: 
+  id: 3
+  tag_id: 1
+  taggable_id: 1
+  taggable_type: FakeModel
\ No newline at end of file