diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index 67591dd0dc..7803845612 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -109,7 +109,8 @@ module ActiveRecord
         end
 
         def invertible_for?(record)
-          inverse_reflection_for(record)
+          inverse = inverse_reflection_for(record)
+          inverse && (inverse.has_one? || ActiveRecord::Base.has_many_inversing)
         end
 
         def stale_state
diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 0cf1704bdf..3ad44d6571 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -286,6 +286,8 @@ module ActiveRecord
       end
 
       def target=(record)
+        return super unless ActiveRecord::Base.has_many_inversing
+
         case record
         when Array
           super
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index f40c912cc1..435416055f 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -128,6 +128,8 @@ module ActiveRecord
 
       mattr_accessor :reading_role, instance_accessor: false, default: :reading
 
+      mattr_accessor :has_many_inversing, instance_accessor: false, default: false
+
       class_attribute :default_connection_handler, instance_writer: false
 
       self.filter_attributes = []
diff --git a/activerecord/lib/active_record/railtie.rb b/activerecord/lib/active_record/railtie.rb
index c0998e8bc6..df5b34480f 100644
--- a/activerecord/lib/active_record/railtie.rb
+++ b/activerecord/lib/active_record/railtie.rb
@@ -28,6 +28,7 @@ module ActiveRecord
 
     config.active_record.use_schema_cache_dump = true
     config.active_record.maintain_test_schema = true
+    config.active_record.has_many_inversing = false
 
     config.active_record.sqlite3 = ActiveSupport::OrderedOptions.new
     config.active_record.sqlite3.represent_boolean_as_integer = nil
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index cf3e7c164c..9f5270e580 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -607,7 +607,7 @@ class InverseBelongsToTests < ActiveRecord::TestCase
     assert_equal f.description, m.face.description, "Description of face should be the same after changes to newly-created-parent-owned instance"
   end
 
-  def test_should_try_to_set_inverse_instances_when_the_inverse_is_a_has_many
+  def test_should_not_try_to_set_inverse_instances_when_the_inverse_is_a_has_many
     i = interests(:trainspotting)
     m = i.man
     assert_not_nil m.interests
@@ -615,14 +615,31 @@ class InverseBelongsToTests < ActiveRecord::TestCase
     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_equal i.topic, iz.topic, "Interest topics should be the same after changes to child"
+    assert_not_equal i.topic, iz.topic, "Interest topics should not be the same after changes to child"
     iz.topic = "Cow tipping"
-    assert_equal i.topic, iz.topic, "Interest topics should be the same after changes to parent-owned instance"
+    assert_not_equal i.topic, iz.topic, "Interest topics should not be the same after changes to parent-owned instance"
   end
 
-  def test_does_not_trigger_association_callbacks_on_set_when_the_inverse_is_a_has_many
-    man = interests(:trainspotting).man_with_callbacks
-    assert_not_predicate man, :add_callback_called?
+  def test_with_has_many_inversing_should_try_to_set_inverse_instances_when_the_inverse_is_a_has_many
+    with_has_many_inversing do
+      i = interests(:trainspotting)
+      m = i.man
+      assert_not_nil m.interests
+      iz = m.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_equal i.topic, iz.topic, "Interest topics should be the same after changes to child"
+      iz.topic = "Cow tipping"
+      assert_equal i.topic, iz.topic, "Interest topics should be the same after changes to parent-owned instance"
+    end
+  end
+
+  def test_with_has_many_inversing_does_not_trigger_association_callbacks_on_set_when_the_inverse_is_a_has_many
+    with_has_many_inversing do
+      man = interests(:trainspotting).man_with_callbacks
+      assert_not_predicate man, :add_callback_called?
+    end
   end
 
   def test_child_instance_should_be_shared_with_replaced_via_accessor_parent
@@ -709,7 +726,7 @@ class InversePolymorphicBelongsToTests < ActiveRecord::TestCase
     assert_equal old_inversed_man.object_id, new_inversed_man.object_id
   end
 
-  def test_should_try_to_set_inverse_instances_when_the_inverse_is_a_has_many
+  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
@@ -717,14 +734,31 @@ class InversePolymorphicBelongsToTests < ActiveRecord::TestCase
     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_equal i.topic, iz.topic, "Interest topics should be the same after changes to child"
+    assert_not_equal i.topic, iz.topic, "Interest topics should not be the same after changes to child"
     iz.topic = "Cow tipping"
-    assert_equal i.topic, iz.topic, "Interest topics should be the same after changes to parent-owned instance"
+    assert_not_equal i.topic, iz.topic, "Interest topics should not be the same after changes to parent-owned instance"
   end
 
-  def test_does_not_trigger_association_callbacks_on_set_when_the_inverse_is_a_has_many
-    man = interests(:llama_wrangling).polymorphic_man_with_callbacks
-    assert_not_predicate man, :add_callback_called?
+  def test_with_has_many_inversing_should_try_to_set_inverse_instances_when_the_inverse_is_a_has_many
+    with_has_many_inversing do
+      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_equal i.topic, iz.topic, "Interest topics should be the same after changes to child"
+      iz.topic = "Cow tipping"
+      assert_equal i.topic, iz.topic, "Interest topics should be the same after changes to parent-owned instance"
+    end
+  end
+
+  def test_with_has_many_inversing_does_not_trigger_association_callbacks_on_set_when_the_inverse_is_a_has_many
+    with_has_many_inversing do
+      man = interests(:llama_wrangling).polymorphic_man_with_callbacks
+      assert_not_predicate man, :add_callback_called?
+    end
   end
 
   def test_trying_to_access_inverses_that_dont_exist_shouldnt_raise_an_error
diff --git a/activerecord/test/cases/test_case.rb b/activerecord/test/cases/test_case.rb
index 1b8bad32a4..9696d407d8 100644
--- a/activerecord/test/cases/test_case.rb
+++ b/activerecord/test/cases/test_case.rb
@@ -79,6 +79,14 @@ module ActiveRecord
       model.reset_column_information
       model.column_names.include?(column_name.to_s)
     end
+
+    def with_has_many_inversing
+      old = ActiveRecord::Base.has_many_inversing
+      ActiveRecord::Base.has_many_inversing = true
+      yield
+    ensure
+      ActiveRecord::Base.has_many_inversing = old
+    end
   end
 
   class PostgreSQLTestCase < TestCase
diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index 934578e9f1..93d3c430b5 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/railties/lib/rails/application/configuration.rb
@@ -154,6 +154,10 @@ module Rails
           end
         when "6.1"
           load_defaults "6.0"
+
+          if respond_to?(:active_record)
+            active_record.has_many_inversing = true
+          end
         else
           raise "Unknown version #{target_version.to_s.inspect}"
         end
diff --git a/railties/lib/rails/generators/rails/app/app_generator.rb b/railties/lib/rails/generators/rails/app/app_generator.rb
index 27749ce3a1..1dfffca3ba 100644
--- a/railties/lib/rails/generators/rails/app/app_generator.rb
+++ b/railties/lib/rails/generators/rails/app/app_generator.rb
@@ -477,7 +477,7 @@ module Rails
 
       def delete_new_framework_defaults
         unless options[:update]
-          remove_file "config/initializers/new_framework_defaults_6_0.rb"
+          remove_file "config/initializers/new_framework_defaults_6_1.rb"
         end
       end
 
diff --git a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
deleted file mode 100644
index 92240ef5f5..0000000000
--- a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
+++ /dev/null
@@ -1,45 +0,0 @@
-# Be sure to restart your server when you modify this file.
-#
-# This file contains migration options to ease your Rails 6.0 upgrade.
-#
-# Once upgraded flip defaults one by one to migrate to the new default.
-#
-# Read the Guide for Upgrading Ruby on Rails for more info on each option.
-
-# Don't force requests from old versions of IE to be UTF-8 encoded.
-# Rails.application.config.action_view.default_enforce_utf8 = false
-
-# Embed purpose and expiry metadata inside signed and encrypted
-# cookies for increased security.
-#
-# This option is not backwards compatible with earlier Rails versions.
-# It's best enabled when your entire app is migrated and stable on 6.0.
-# Rails.application.config.action_dispatch.use_cookies_with_metadata = true
-
-# Change the return value of `ActionDispatch::Response#content_type` to Content-Type header without modification.
-# Rails.application.config.action_dispatch.return_only_media_type_on_content_type = false
-
-# Return false instead of self when enqueuing is aborted from a callback.
-# Rails.application.config.active_job.return_false_on_aborted_enqueue = true
-
-# Send Active Storage analysis and purge jobs to dedicated queues.
-# Rails.application.config.active_storage.queues.analysis = :active_storage_analysis
-# Rails.application.config.active_storage.queues.purge    = :active_storage_purge
-
-# When assigning to a collection of attachments declared via `has_many_attached`, replace existing
-# attachments instead of appending. Use #attach to add new attachments without replacing existing ones.
-# Rails.application.config.active_storage.replace_on_assign_to_many = true
-
-# Use ActionMailer::MailDeliveryJob for sending parameterized and normal mail.
-#
-# The default delivery jobs (ActionMailer::Parameterized::DeliveryJob, ActionMailer::DeliveryJob),
-# will be removed in Rails 6.1. This setting is not backwards compatible with earlier Rails versions.
-# If you send mail in the background, job workers need to have a copy of
-# MailDeliveryJob to ensure all delivery jobs are processed properly.
-# Make sure your entire app is migrated and stable on 6.0 before using this setting.
-# Rails.application.config.action_mailer.delivery_job = "ActionMailer::MailDeliveryJob"
-
-# Enable the same cache key to be reused when the object being cached of type
-# `ActiveRecord::Relation` changes by moving the volatile information (max updated at and count)
-# of the relation's cache key into the cache version to support recycling cache key.
-# Rails.application.config.active_record.collection_cache_versioning = true
diff --git a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_1.rb.tt b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_1.rb.tt
new file mode 100644
index 0000000000..c3bf0aa145
--- /dev/null
+++ b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_1.rb.tt
@@ -0,0 +1,10 @@
+# Be sure to restart your server when you modify this file.
+#
+# This file contains migration options to ease your Rails 6.1 upgrade.
+#
+# Once upgraded flip defaults one by one to migrate to the new default.
+#
+# Read the Guide for Upgrading Ruby on Rails for more info on each option.
+
+# Support for inversing belongs_to -> has_many Active Record associations.
+# Rails.application.config.active_record.has_many_inversing = true
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index 90da870233..bacafbc810 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -2144,42 +2144,6 @@ module ApplicationTests
       assert_equal({}, Rails.application.config.my_custom_config)
     end
 
-    test "represent_boolean_as_integer is deprecated" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.active_record.sqlite3.represent_boolean_as_integer = true
-      RUBY
-
-      app_file "app/models/post.rb", <<-RUBY
-        class Post < ActiveRecord::Base
-        end
-      RUBY
-
-      app "development"
-      assert_deprecated do
-        force_lazy_load_hooks { Post }
-      end
-    end
-
-    test "represent_boolean_as_integer raises when the value is false" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.active_record.sqlite3.represent_boolean_as_integer = false
-      RUBY
-
-      app_file "app/models/post.rb", <<-RUBY
-        class Post < ActiveRecord::Base
-        end
-      RUBY
-
-      app "development"
-      assert_raises(RuntimeError) do
-        force_lazy_load_hooks { Post }
-      end
-    end
-
     test "config_for containing ERB tags should evaluate" do
       app_file "config/custom.yml", <<-RUBY
       development:
@@ -2287,6 +2251,18 @@ module ApplicationTests
       assert_equal "https://example.org/", last_response.location
     end
 
+    test "ActiveSupport::MessageEncryptor.use_authenticated_message_encryption can be configured via config.active_support.use_authenticated_message_encryption" do
+      remove_from_config '.*config\.load_defaults.*\n'
+
+      app_file "config/initializers/new_framework_defaults_6_1.rb", <<-RUBY
+        Rails.application.config.active_record.has_many_inversing = true
+      RUBY
+
+      app "development"
+
+      assert_equal true, ActiveRecord::Base.has_many_inversing
+    end
+
     test "ActiveSupport::MessageEncryptor.use_authenticated_message_encryption is true by default for new apps" do
       app "development"
 
@@ -2301,18 +2277,6 @@ module ApplicationTests
       assert_equal false, ActiveSupport::MessageEncryptor.use_authenticated_message_encryption
     end
 
-    test "ActiveSupport::MessageEncryptor.use_authenticated_message_encryption can be configured via config.active_support.use_authenticated_message_encryption" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.active_support.use_authenticated_message_encryption = true
-      RUBY
-
-      app "development"
-
-      assert_equal true, ActiveSupport::MessageEncryptor.use_authenticated_message_encryption
-    end
-
     test "ActiveSupport::Digest.hash_digest_class is Digest::SHA1 by default for new apps" do
       app "development"
 
@@ -2327,18 +2291,6 @@ module ApplicationTests
       assert_equal Digest::MD5, ActiveSupport::Digest.hash_digest_class
     end
 
-    test "ActiveSupport::Digest.hash_digest_class can be configured via config.active_support.use_sha1_digests" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.active_support.use_sha1_digests = true
-      RUBY
-
-      app "development"
-
-      assert_equal Digest::SHA1, ActiveSupport::Digest.hash_digest_class
-    end
-
     test "custom serializers should be able to set via config.active_job.custom_serializers in an initializer" do
       class ::DummySerializer < ActiveJob::Serializers::ObjectSerializer; end
 
@@ -2365,18 +2317,6 @@ module ApplicationTests
       assert_equal true, ActionView::Helpers::FormTagHelper.default_enforce_utf8
     end
 
-    test "ActionView::Helpers::FormTagHelper.default_enforce_utf8 can be configured via config.action_view.default_enforce_utf8" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.action_view.default_enforce_utf8 = true
-      RUBY
-
-      app "development"
-
-      assert_equal true, ActionView::Helpers::FormTagHelper.default_enforce_utf8
-    end
-
     test "ActionView::Template.finalize_compiled_template_methods is true by default" do
       app "test"
       assert_deprecated do
@@ -2413,18 +2353,6 @@ module ApplicationTests
       assert_equal false, ActiveJob::Base.return_false_on_aborted_enqueue
     end
 
-    test "ActiveJob::Base.return_false_on_aborted_enqueue can be configured in the new framework defaults" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.active_job.return_false_on_aborted_enqueue = true
-      RUBY
-
-      app "development"
-
-      assert_equal true, ActiveJob::Base.return_false_on_aborted_enqueue
-    end
-
     test "ActiveStorage.queues[:analysis] is :active_storage_analysis by default" do
       app "development"
 
@@ -2468,18 +2396,6 @@ module ApplicationTests
       assert_equal true, ActionDispatch::Response.return_only_media_type_on_content_type
     end
 
-    test "ActionDispatch::Response.return_only_media_type_on_content_type can be configured in the new framework defaults" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.action_dispatch.return_only_media_type_on_content_type = false
-      RUBY
-
-      app "development"
-
-      assert_equal false, ActionDispatch::Response.return_only_media_type_on_content_type
-    end
-
     test "ActionMailbox.logger is Rails.logger by default" do
       app "development"
 
@@ -2566,18 +2482,6 @@ module ApplicationTests
       assert_equal ActionMailer::DeliveryJob, ActionMailer::Base.delivery_job
     end
 
-    test "ActionMailer::Base.delivery_job can be configured in the new framework defaults" do
-      remove_from_config '.*config\.load_defaults.*\n'
-
-      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
-        Rails.application.config.action_mailer.delivery_job = "ActionMailer::MailDeliveryJob"
-      RUBY
-
-      app "development"
-
-      assert_equal ActionMailer::MailDeliveryJob, ActionMailer::Base.delivery_job
-    end
-
     test "ActiveRecord::Base.filter_attributes should equal to filter_parameters" do
       app_file "config/initializers/filter_parameters_logging.rb", <<-RUBY
         Rails.application.config.filter_parameters += [ :password, :credit_card_number ]
diff --git a/railties/test/generators/app_generator_test.rb b/railties/test/generators/app_generator_test.rb
index 690b5cb2ab..afb61089de 100644
--- a/railties/test/generators/app_generator_test.rb
+++ b/railties/test/generators/app_generator_test.rb
@@ -212,7 +212,7 @@ class AppGeneratorTest < Rails::Generators::TestCase
 
   def test_new_application_doesnt_need_defaults
     run_generator
-    assert_no_file "config/initializers/new_framework_defaults_6_0.rb"
+    assert_no_file "config/initializers/new_framework_defaults_6_1.rb"
   end
 
   def test_new_application_load_defaults
@@ -269,14 +269,14 @@ class AppGeneratorTest < Rails::Generators::TestCase
     app_root = File.join(destination_root, "myapp")
     run_generator [app_root]
 
-    assert_no_file "#{app_root}/config/initializers/new_framework_defaults_6_0.rb"
+    assert_no_file "#{app_root}/config/initializers/new_framework_defaults_6_1.rb"
 
     stub_rails_application(app_root) do
       generator = Rails::Generators::AppGenerator.new ["rails"], { update: true }, { destination_root: app_root, shell: @shell }
       generator.send(:app_const)
       quietly { generator.send(:update_config_files) }
 
-      assert_file "#{app_root}/config/initializers/new_framework_defaults_6_0.rb"
+      assert_file "#{app_root}/config/initializers/new_framework_defaults_6_1.rb"
     end
   end