diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 337fc8a35023c63c8b47be27d0f3d2e848b10821..b12af5bdb67deb59cbefcbcc2ad2d8484cbfd2d4 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,12 @@
+*   Support storing demodulized class name for polymorphic type.
+
+    Before Rails 6.1, storing demodulized class name is supported only for STI type
+    by `store_full_sti_class` class attribute.
+
+    Now `store_full_class_name` class attribute can handle both STI and polymorphic types.
+
+    *Ryuta Kamizono*
+
 *   Deprecate `rails db:structure:{load, dump}` tasks and extend
     `rails db:schema:{load, dump}` tasks to work with either `:ruby` or `:sql` format,
     depending on `config.active_record.schema_format` configuration value.
diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index c38dd10bf339e0f1debe5f1907ae4cfb959434b7..d0ef768433f9cf9e626d5f9893e79b1b110b6fae 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -38,6 +38,8 @@ module Inheritance
     extend ActiveSupport::Concern
 
     included do
+      class_attribute :store_full_class_name, instance_writer: false, default: true
+
       # Determines whether to store the full constant name including namespace when using STI.
       # This is true, by default.
       class_attribute :store_full_sti_class, instance_writer: false, default: true
@@ -164,14 +166,14 @@ def abstract_class?
 
       # Returns the value to be stored in the inheritance column for STI.
       def sti_name
-        store_full_sti_class ? name : name.demodulize
+        store_full_sti_class && store_full_class_name ? name : name.demodulize
       end
 
       # Returns the class for the provided +type_name+.
       #
       # It is used to find the class correspondent to the value stored in the inheritance column.
       def sti_class_for(type_name)
-        if store_full_sti_class
+        if store_full_sti_class && store_full_class_name
           ActiveSupport::Dependencies.constantize(type_name)
         else
           compute_type(type_name)
@@ -186,14 +188,18 @@ def sti_class_for(type_name)
 
       # Returns the value to be stored in the polymorphic type column for Polymorphic Associations.
       def polymorphic_name
-        base_class.name
+        store_full_class_name ? base_class.name : base_class.name.demodulize
       end
 
       # Returns the class for the provided +name+.
       #
       # It is used to find the class correspondent to the value stored in the polymorphic type column.
       def polymorphic_class_for(name)
-        name.constantize
+        if store_full_class_name
+          ActiveSupport::Dependencies.constantize(name)
+        else
+          compute_type(name)
+        end
       end
 
       def inherited(subclass)
diff --git a/activerecord/test/cases/associations/eager_load_includes_full_sti_class_test.rb b/activerecord/test/cases/associations/eager_load_includes_full_sti_class_test.rb
index 673d5f1dcf03d21ba5b64d06208f0ccad29393d4..c5cc94bb05d01391384b832dcebc4ed2f4ffa1ba 100644
--- a/activerecord/test/cases/associations/eager_load_includes_full_sti_class_test.rb
+++ b/activerecord/test/cases/associations/eager_load_includes_full_sti_class_test.rb
@@ -8,14 +8,10 @@ module Namespaced
   class Post < ActiveRecord::Base
     self.table_name = "posts"
     has_one :tagging, as: :taggable, class_name: "Tagging"
-
-    def self.polymorphic_name
-      sti_name
-    end
   end
 end
 
-module PolymorphicFullStiClassNamesSharedTest
+module FullStiClassNamesSharedTest
   def setup
     @old_store_full_sti_class = ActiveRecord::Base.store_full_sti_class
     ActiveRecord::Base.store_full_sti_class = store_full_sti_class
@@ -31,7 +27,7 @@ def teardown
   def test_class_names
     ActiveRecord::Base.store_full_sti_class = !store_full_sti_class
     post = Namespaced::Post.find_by_title("Great stuff")
-    assert_nil post.tagging
+    assert_equal @tagging, post.tagging
 
     ActiveRecord::Base.store_full_sti_class = store_full_sti_class
     post = Namespaced::Post.find_by_title("Great stuff")
@@ -41,7 +37,7 @@ def test_class_names
   def test_class_names_with_includes
     ActiveRecord::Base.store_full_sti_class = !store_full_sti_class
     post = Namespaced::Post.includes(:tagging).find_by_title("Great stuff")
-    assert_nil post.tagging
+    assert_equal @tagging, post.tagging
 
     ActiveRecord::Base.store_full_sti_class = store_full_sti_class
     post = Namespaced::Post.includes(:tagging).find_by_title("Great stuff")
@@ -51,7 +47,7 @@ def test_class_names_with_includes
   def test_class_names_with_eager_load
     ActiveRecord::Base.store_full_sti_class = !store_full_sti_class
     post = Namespaced::Post.eager_load(:tagging).find_by_title("Great stuff")
-    assert_nil post.tagging
+    assert_equal @tagging, post.tagging
 
     ActiveRecord::Base.store_full_sti_class = store_full_sti_class
     post = Namespaced::Post.eager_load(:tagging).find_by_title("Great stuff")
@@ -62,15 +58,15 @@ def test_class_names_with_find_by
     post = Namespaced::Post.find_by_title("Great stuff")
 
     ActiveRecord::Base.store_full_sti_class = !store_full_sti_class
-    assert_nil Tagging.find_by(taggable: post)
+    assert_equal @tagging, Tagging.find_by(taggable: post)
 
     ActiveRecord::Base.store_full_sti_class = store_full_sti_class
     assert_equal @tagging, Tagging.find_by(taggable: post)
   end
 end
 
-class PolymorphicFullStiClassNamesTest < ActiveRecord::TestCase
-  include PolymorphicFullStiClassNamesSharedTest
+class FullStiClassNamesTest < ActiveRecord::TestCase
+  include FullStiClassNamesSharedTest
 
   private
     def store_full_sti_class
@@ -78,11 +74,83 @@ def store_full_sti_class
     end
 end
 
-class PolymorphicNonFullStiClassNamesTest < ActiveRecord::TestCase
-  include PolymorphicFullStiClassNamesSharedTest
+class NonFullStiClassNamesTest < ActiveRecord::TestCase
+  include FullStiClassNamesSharedTest
 
   private
     def store_full_sti_class
       false
     end
 end
+
+module PolymorphicFullClassNamesSharedTest
+  def setup
+    @old_store_full_class_name = ActiveRecord::Base.store_full_class_name
+    ActiveRecord::Base.store_full_class_name = store_full_class_name
+
+    post = Namespaced::Post.create(title: "Great stuff", body: "This is not", author_id: 1)
+    @tagging = post.create_tagging!
+  end
+
+  def teardown
+    ActiveRecord::Base.store_full_class_name = @old_store_full_class_name
+  end
+
+  def test_class_names
+    ActiveRecord::Base.store_full_class_name = !store_full_class_name
+    post = Namespaced::Post.find_by_title("Great stuff")
+    assert_nil post.tagging
+
+    ActiveRecord::Base.store_full_class_name = store_full_class_name
+    post = Namespaced::Post.find_by_title("Great stuff")
+    assert_equal @tagging, post.tagging
+  end
+
+  def test_class_names_with_includes
+    ActiveRecord::Base.store_full_class_name = !store_full_class_name
+    post = Namespaced::Post.includes(:tagging).find_by_title("Great stuff")
+    assert_nil post.tagging
+
+    ActiveRecord::Base.store_full_class_name = store_full_class_name
+    post = Namespaced::Post.includes(:tagging).find_by_title("Great stuff")
+    assert_equal @tagging, post.tagging
+  end
+
+  def test_class_names_with_eager_load
+    ActiveRecord::Base.store_full_class_name = !store_full_class_name
+    post = Namespaced::Post.eager_load(:tagging).find_by_title("Great stuff")
+    assert_nil post.tagging
+
+    ActiveRecord::Base.store_full_class_name = store_full_class_name
+    post = Namespaced::Post.eager_load(:tagging).find_by_title("Great stuff")
+    assert_equal @tagging, post.tagging
+  end
+
+  def test_class_names_with_find_by
+    post = Namespaced::Post.find_by_title("Great stuff")
+
+    ActiveRecord::Base.store_full_class_name = !store_full_class_name
+    assert_nil Tagging.find_by(taggable: post)
+
+    ActiveRecord::Base.store_full_class_name = store_full_class_name
+    assert_equal @tagging, Tagging.find_by(taggable: post)
+  end
+end
+
+class PolymorphicFullClassNamesTest < ActiveRecord::TestCase
+  include PolymorphicFullClassNamesSharedTest
+
+  private
+    def store_full_class_name
+      true
+    end
+end
+
+class PolymorphicNonFullClassNamesTest < ActiveRecord::TestCase
+  include PolymorphicFullClassNamesSharedTest
+
+  private
+    def store_full_class_name
+      false
+    end
+end