diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 35f7c158a604d3a096f6ed656cd7e79222f84f8c..64a0e22af238ddb6e8043aefa3ce893f0c0ce729 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Fix the uniqueness validation scope with a polymorphic association.
+
+    *Sergey Alekseev*
+
 *   Raise ActiveRecord::RecordNotFound from collection `*_ids` setters
     for unknown IDs with a better error message.
 
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index bed93bfc267db83e9c4e2c03d9c2312c2c2e67d5..512fdadacccf75260d77b2faf080fa2a2dac8195 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -85,11 +85,10 @@ def build_relation(klass, attribute, value) # :nodoc:
 
       def scope_relation(record, relation)
         Array(options[:scope]).each do |scope_item|
-          if reflection = record.class._reflect_on_association(scope_item)
-            scope_value = record.send(reflection.foreign_key)
-            scope_item  = reflection.foreign_key
+          scope_value = if record.class._reflect_on_association(scope_item)
+            record.association(scope_item).reader
           else
-            scope_value = record._read_attribute(scope_item)
+            record._read_attribute(scope_item)
           end
           relation = relation.where(scope_item => scope_value)
         end
diff --git a/activerecord/test/cases/validations/uniqueness_validation_test.rb b/activerecord/test/cases/validations/uniqueness_validation_test.rb
index 44b4e28777f2a4af9d3b9d136e2ebc0333513958..6d226385920b3299b74cd5b6f6434827f41ac161 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/activerecord/test/cases/validations/uniqueness_validation_test.rb
@@ -6,6 +6,9 @@
 require "models/event"
 require "models/dashboard"
 require "models/uuid_item"
+require "models/author"
+require "models/person"
+require "models/essay"
 
 class Wizard < ActiveRecord::Base
   self.abstract_class = true
@@ -163,6 +166,19 @@ def test_validate_uniqueness_with_object_scope
     assert !r2.valid?, "Saving r2 first time"
   end
 
+  def test_validate_uniqueness_with_polymorphic_object_scope
+    Essay.validates_uniqueness_of(:name, scope: :writer)
+
+    a = Author.create(name: "Sergey")
+    p = Person.create(first_name: "Sergey")
+
+    e1 = a.essays.create(name: "Essay")
+    assert e1.valid?, "Saving e1"
+
+    e2 = p.essays.create(name: "Essay")
+    assert e2.valid?, "Saving e2"
+  end
+
   def test_validate_uniqueness_with_composed_attribute_scope
     r1 = ReplyWithTitleObject.create "title" => "r1", "content" => "hello world"
     assert r1.valid?, "Saving r1"