diff --git a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
index 67e18d692dc15..9678300003e9e 100644
--- a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
@@ -13,6 +13,7 @@ def replace(record)
           @updated = true
         end
 
+        set_inverse_instance(record, @owner)
         loaded
         record
       end
@@ -22,19 +23,37 @@ def updated?
       end
 
       private
+
+        # NOTE - for now, we're only supporting inverse setting from belongs_to back onto
+        # has_one associations.
+        def we_can_set_the_inverse_on_this?(record)
+          @reflection.has_inverse? && @reflection.polymorphic_inverse_of(record.class).macro == :has_one
+        end
+
+        def set_inverse_instance(record, instance)
+          return if record.nil? || !we_can_set_the_inverse_on_this?(record)
+          inverse_relationship = @reflection.polymorphic_inverse_of(record.class)
+          unless inverse_relationship.nil?
+            record.send(:"set_#{inverse_relationship.name}_target", instance)
+          end
+        end
+
         def find_target
           return nil if association_class.nil?
 
-          if @reflection.options[:conditions]
-            association_class.find(
-              @owner[@reflection.primary_key_name],
-              :select     => @reflection.options[:select],
-              :conditions => conditions,
-              :include    => @reflection.options[:include]
-            )
-          else
-            association_class.find(@owner[@reflection.primary_key_name], :select => @reflection.options[:select], :include => @reflection.options[:include])
-          end
+          target =
+            if @reflection.options[:conditions]
+              association_class.find(
+                @owner[@reflection.primary_key_name],
+                :select     => @reflection.options[:select],
+                :conditions => conditions,
+                :include    => @reflection.options[:include]
+              )
+            else
+              association_class.find(@owner[@reflection.primary_key_name], :select => @reflection.options[:select], :include => @reflection.options[:include])
+            end
+          set_inverse_instance(target, @owner) if target
+          target
         end
 
         def foreign_key_present
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index db5d2b25ed486..72f7df32c738a 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -214,8 +214,10 @@ def check_validity!
       end
 
       def check_validity_of_inverse!
-        if has_inverse? && inverse_of.nil?
-          raise InverseOfAssociationNotFoundError.new(self)
+        unless options[:polymorphic]
+          if has_inverse? && inverse_of.nil?
+            raise InverseOfAssociationNotFoundError.new(self)
+          end
         end
       end
 
@@ -242,6 +244,14 @@ def inverse_of
         end
       end
 
+      def polymorphic_inverse_of(associated_class)
+        if has_inverse?
+          associated_class.reflect_on_association(options[:inverse_of])
+        else
+          nil
+        end
+      end
+
       private
         def derive_class_name
           class_name = name.to_s.camelize
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index c3d0d61ceced5..2ab3aa141a081 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -85,7 +85,7 @@ class InverseHasOneTests < ActiveRecord::TestCase
   fixtures :men, :faces
 
   def test_parent_instance_should_be_shared_with_child_on_find
-    m = Man.find(:first)
+    m = men(:gordon)
     f = m.face
     assert_equal m.name, f.man.name, "Name of man should be the same before changes to parent instance"
     m.name = 'Bongo'
@@ -96,7 +96,7 @@ def test_parent_instance_should_be_shared_with_child_on_find
 
 
   def test_parent_instance_should_be_shared_with_eager_loaded_child_on_find
-    m = Man.find(:first, :include => :face)
+    m = Man.find(:first, :conditions => {:name => 'Gordon'}, :include => :face)
     f = m.face
     assert_equal m.name, f.man.name, "Name of man should be the same before changes to parent instance"
     m.name = 'Bongo'
@@ -104,7 +104,7 @@ def test_parent_instance_should_be_shared_with_eager_loaded_child_on_find
     f.man.name = 'Mungo'
     assert_equal m.name, f.man.name, "Name of man should be the same after changes to child-owned instance"
 
-    m = Man.find(:first, :include => :face, :order => 'faces.id')
+    m = Man.find(:first, :conditions => {:name => 'Gordon'}, :include => :face, :order => 'faces.id')
     f = m.face
     assert_equal m.name, f.man.name, "Name of man should be the same before changes to parent instance"
     m.name = 'Bongo'
@@ -114,7 +114,7 @@ def test_parent_instance_should_be_shared_with_eager_loaded_child_on_find
   end
 
   def test_parent_instance_should_be_shared_with_newly_built_child
-    m = Man.find(:first)
+    m = men(:gordon)
     f = m.build_face(:description => 'haunted')
     assert_not_nil f.man
     assert_equal m.name, f.man.name, "Name of man should be the same before changes to parent instance"
@@ -125,7 +125,7 @@ def test_parent_instance_should_be_shared_with_newly_built_child
   end
 
   def test_parent_instance_should_be_shared_with_newly_created_child
-    m = Man.find(:first)
+    m = men(:gordon)
     f = m.create_face(:description => 'haunted')
     assert_not_nil f.man
     assert_equal m.name, f.man.name, "Name of man should be the same before changes to parent instance"
@@ -224,7 +224,7 @@ class InverseHasManyTests < ActiveRecord::TestCase
   fixtures :men, :interests
 
   def test_parent_instance_should_be_shared_with_every_child_on_find
-    m = Man.find(:first)
+    m = men(:gordon)
     is = m.interests
     is.each do |i|
       assert_equal m.name, i.man.name, "Name of man should be the same before changes to parent instance"
@@ -236,7 +236,7 @@ def test_parent_instance_should_be_shared_with_every_child_on_find
   end
 
   def test_parent_instance_should_be_shared_with_eager_loaded_children
-    m = Man.find(:first, :include => :interests)
+    m = Man.find(:first, :conditions => {:name => 'Gordon'}, :include => :interests)
     is = m.interests
     is.each do |i|
       assert_equal m.name, i.man.name, "Name of man should be the same before changes to parent instance"
@@ -246,7 +246,7 @@ def test_parent_instance_should_be_shared_with_eager_loaded_children
       assert_equal m.name, i.man.name, "Name of man should be the same after changes to child-owned instance"
     end
 
-    m = Man.find(:first, :include => :interests, :order => 'interests.id')
+    m = Man.find(:first, :conditions => {:name => 'Gordon'}, :include => :interests, :order => 'interests.id')
     is = m.interests
     is.each do |i|
       assert_equal m.name, i.man.name, "Name of man should be the same before changes to parent instance"
@@ -255,11 +255,10 @@ def test_parent_instance_should_be_shared_with_eager_loaded_children
       i.man.name = 'Mungo'
       assert_equal m.name, i.man.name, "Name of man should be the same after changes to child-owned instance"
     end
-
   end
 
   def test_parent_instance_should_be_shared_with_newly_built_child
-    m = Man.find(:first)
+    m = men(:gordon)
     i = m.interests.build(:topic => 'Industrial Revolution Re-enactment')
     assert_not_nil i.man
     assert_equal m.name, i.man.name, "Name of man should be the same before changes to parent instance"
@@ -282,7 +281,7 @@ def test_parent_instance_should_be_shared_with_newly_block_style_built_child
   end
 
   def test_parent_instance_should_be_shared_with_newly_created_child
-    m = Man.find(:first)
+    m = men(:gordon)
     i = m.interests.create(:topic => 'Industrial Revolution Re-enactment')
     assert_not_nil i.man
     assert_equal m.name, i.man.name, "Name of man should be the same before changes to parent instance"
@@ -316,7 +315,7 @@ def test_parent_instance_should_be_shared_with_newly_block_style_created_child
   end
 
   def test_parent_instance_should_be_shared_with_poked_in_child
-    m = Man.find(:first)
+    m = men(:gordon)
     i = Interest.create(:topic => 'Industrial Revolution Re-enactment')
     m.interests << i
     assert_not_nil i.man
@@ -360,7 +359,7 @@ class InverseBelongsToTests < ActiveRecord::TestCase
   fixtures :men, :faces, :interests
 
   def test_child_instance_should_be_shared_with_parent_on_find
-    f = Face.find(:first)
+    f = faces(:trusting)
     m = f.man
     assert_equal f.description, m.face.description, "Description of face should be the same before changes to child instance"
     f.description = 'gormless'
@@ -370,7 +369,7 @@ def test_child_instance_should_be_shared_with_parent_on_find
   end
 
   def test_eager_loaded_child_instance_should_be_shared_with_parent_on_find
-    f = Face.find(:first, :include => :man)
+    f = Face.find(:first, :include => :man, :conditions => {:description => 'trusting'})
     m = f.man
     assert_equal f.description, m.face.description, "Description of face should be the same before changes to child instance"
     f.description = 'gormless'
@@ -378,8 +377,7 @@ def test_eager_loaded_child_instance_should_be_shared_with_parent_on_find
     m.face.description = 'pleasing'
     assert_equal f.description, m.face.description, "Description of face should be the same after changes to parent-owned instance"
 
-
-    f = Face.find(:first, :include => :man, :order => 'men.id')
+    f = Face.find(:first, :include => :man, :order => 'men.id', :conditions => {:description => 'trusting'})
     m = f.man
     assert_equal f.description, m.face.description, "Description of face should be the same before changes to child instance"
     f.description = 'gormless'
@@ -389,7 +387,7 @@ def test_eager_loaded_child_instance_should_be_shared_with_parent_on_find
   end
 
   def test_child_instance_should_be_shared_with_newly_built_parent
-    f = Face.find(:first)
+    f = faces(:trusting)
     m = f.build_man(:name => 'Charles')
     assert_not_nil m.face
     assert_equal f.description, m.face.description, "Description of face should be the same before changes to child instance"
@@ -400,7 +398,7 @@ def test_child_instance_should_be_shared_with_newly_built_parent
   end
 
   def test_child_instance_should_be_shared_with_newly_created_parent
-    f = Face.find(:first)
+    f = faces(:trusting)
     m = f.create_man(:name => 'Charles')
     assert_not_nil m.face
     assert_equal f.description, m.face.description, "Description of face should be the same before changes to child instance"
@@ -411,7 +409,7 @@ def test_child_instance_should_be_shared_with_newly_created_parent
   end
 
   def test_should_not_try_to_set_inverse_instances_when_the_inverse_is_a_has_many
-    i = Interest.find(:first)
+    i = interests(:trainspotting)
     m = i.man
     assert_not_nil m.interests
     iz = m.interests.detect {|iz| iz.id == i.id}
@@ -452,6 +450,70 @@ def test_trying_to_use_inverses_that_dont_exist_should_raise_an_error
   end
 end
 
+class InversePolymorphicBelongsToTests < ActiveRecord::TestCase
+  fixtures :men, :faces, :interests
+
+  def test_child_instance_should_be_shared_with_parent_on_find
+    f = Face.find(:first, :conditions => {:description => 'confused'})
+    m = f.polymorphic_man
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same before changes to child instance"
+    f.description = 'gormless'
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same after changes to child instance"
+    m.polymorphic_face.description = 'pleasing'
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same after changes to parent-owned instance"
+  end
+
+  def test_eager_loaded_child_instance_should_be_shared_with_parent_on_find
+    f = Face.find(:first, :conditions => {:description => 'confused'}, :include => :man)
+    m = f.polymorphic_man
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same before changes to child instance"
+    f.description = 'gormless'
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same after changes to child instance"
+    m.polymorphic_face.description = 'pleasing'
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same after changes to parent-owned instance"
+
+    f = Face.find(:first, :conditions => {:description => 'confused'}, :include => :man, :order => 'men.id')
+    m = f.polymorphic_man
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same before changes to child instance"
+    f.description = 'gormless'
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same after changes to child instance"
+    m.polymorphic_face.description = 'pleasing'
+    assert_equal f.description, m.polymorphic_face.description, "Description of face should be the same after changes to parent-owned instance"
+  end
+
+  def test_child_instance_should_be_shared_with_replaced_parent
+    face = faces(:confused)
+    old_man = face.polymorphic_man
+    new_man = Man.new
+
+    assert_not_nil face.polymorphic_man
+    face.polymorphic_man.replace(new_man)
+
+    assert_equal face.description, new_man.polymorphic_face.description, "Description of face should be the same before changes to parent instance"
+    face.description = 'Bongo'
+    assert_equal face.description, new_man.polymorphic_face.description, "Description of face should be the same after changes to parent instance"
+    new_man.polymorphic_face.description = 'Mungo'
+    assert_equal face.description, new_man.polymorphic_face.description, "Description of face should be the same after changes to replaced-parent-owned instance"
+  end
+
+  def test_should_not_try_to_set_inverse_instances_when_the_inverse_is_a_has_many
+    i = interests(:llama_wrangling)
+    m = i.polymorphic_man
+    assert_not_nil m.polymorphic_interests
+    iz = m.polymorphic_interests.detect {|iz| iz.id == i.id}
+    assert_not_nil iz
+    assert_equal i.topic, iz.topic, "Interest topics should be the same before changes to child"
+    i.topic = 'Eating cheese with a spoon'
+    assert_not_equal i.topic, iz.topic, "Interest topics should not be the same after changes to child"
+    iz.topic = 'Cow tipping'
+    assert_not_equal i.topic, iz.topic, "Interest topics should not be the same after changes to parent-owned instance"
+  end
+
+  def test_trying_to_use_inverses_that_dont_exist_should_raise_an_error
+    assert_raise(ActiveRecord::InverseOfAssociationNotFoundError) { Face.find(:first).horrible_man }
+  end
+end
+
 # NOTE - these tests might not be meaningful, ripped as they were from the parental_control plugin
 # which would guess the inverse rather than look for an explicit configuration option.
 class InverseMultipleHasManyInversesForSameModel < ActiveRecord::TestCase
diff --git a/activerecord/test/fixtures/faces.yml b/activerecord/test/fixtures/faces.yml
index 1dd2907cf7b89..c8e4a34484e82 100644
--- a/activerecord/test/fixtures/faces.yml
+++ b/activerecord/test/fixtures/faces.yml
@@ -5,3 +5,7 @@ trusting:
 weather_beaten:
   description: weather beaten
   man: steve
+
+confused:
+  description: confused
+  polymorphic_man: gordon (Man)
diff --git a/activerecord/test/fixtures/interests.yml b/activerecord/test/fixtures/interests.yml
index ec71890ab637e..9200a19d5a3d7 100644
--- a/activerecord/test/fixtures/interests.yml
+++ b/activerecord/test/fixtures/interests.yml
@@ -23,7 +23,11 @@ woodsmanship:
   zine: going_out
   man: steve
 
-survial:
+survival:
   topic: Survival
   zine: going_out
   man: steve
+
+llama_wrangling:
+  topic: Llama Wrangling
+  polymorphic_man: gordon (Man)
diff --git a/activerecord/test/models/face.rb b/activerecord/test/models/face.rb
index 1540dbf7412b1..3e2bdc0307eb8 100644
--- a/activerecord/test/models/face.rb
+++ b/activerecord/test/models/face.rb
@@ -1,5 +1,6 @@
 class Face < ActiveRecord::Base
   belongs_to :man, :inverse_of => :face
+  belongs_to :polymorphic_man, :polymorphic => true, :inverse_of => :polymorphic_face
   # This is a "broken" inverse_of for the purposes of testing
   belongs_to :horrible_man, :class_name => 'Man', :inverse_of => :horrible_face
 end
diff --git a/activerecord/test/models/interest.rb b/activerecord/test/models/interest.rb
index d8291d00cc53a..d5d9226204226 100644
--- a/activerecord/test/models/interest.rb
+++ b/activerecord/test/models/interest.rb
@@ -1,4 +1,5 @@
 class Interest < ActiveRecord::Base
   belongs_to :man, :inverse_of => :interests
+  belongs_to :polymorphic_man, :polymorphic => true, :inverse_of => :polymorphic_interests
   belongs_to :zine, :inverse_of => :interests
 end
diff --git a/activerecord/test/models/man.rb b/activerecord/test/models/man.rb
index f40bc9d0fc0a2..4bff92dc98bce 100644
--- a/activerecord/test/models/man.rb
+++ b/activerecord/test/models/man.rb
@@ -1,6 +1,8 @@
 class Man < ActiveRecord::Base
   has_one :face, :inverse_of => :man
+  has_one :polymorphic_face, :class_name => 'Face', :as => :polymorphic_man, :inverse_of => :polymorphic_man
   has_many :interests, :inverse_of => :man
+  has_many :polymorphic_interests, :class_name => 'Interest', :as => :polymorphic_man, :inverse_of => :polymorphic_man
   # These are "broken" inverse_of associations for the purposes of testing
   has_one :dirty_face, :class_name => 'Face', :inverse_of => :dirty_man
   has_many :secret_interests, :class_name => 'Interest', :inverse_of => :secret_man
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 0dd9da4c11fc6..1ec36e7832dff 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -520,11 +520,15 @@ def create_table(*args, &block)
   create_table :faces, :force => true do |t|
     t.string  :description
     t.integer :man_id
+    t.integer :polymorphic_man_id
+    t.string :polymorphic_man_type
   end
 
   create_table :interests, :force => true do |t|
     t.string :topic
     t.integer :man_id
+    t.integer :polymorphic_man_id
+    t.string :polymorphic_man_type
     t.integer :zine_id
   end