diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 44cb8537bd8b0f808b0a18026eb28771f2e8b116..1644f80d743d999e99b9a6e5aff9be06cacefe65 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,12 @@
+*   ActiveRecord's `belongs_to_required_by_default` flag can now be set per model.
+
+    You can now opt-out/opt-in specific models from having their associations required
+    by default.
+    This change is meant to ease the process of migrating all your models to have
+    their association required.
+
+    *Edouard Chin*
+
 *   The `connection_config` method has been deprecated, please use `connection_db_config` instead which will return a `DatabaseConfigurations::DatabaseConfig` instead of a `Hash`.
 
     *Eileen M. Uchitelle*, *John Crepezzi*
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 435416055ffbd1072445f554fc8a48ccf0bbe330..8a97da626370497cbc8cbd64b331dbd017e00af0 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -120,7 +120,7 @@ def self.configurations
 
       mattr_accessor :maintain_test_schema, instance_accessor: false
 
-      mattr_accessor :belongs_to_required_by_default, instance_accessor: false
+      class_attribute :belongs_to_required_by_default, instance_accessor: false
 
       mattr_accessor :connection_handlers, instance_accessor: false, default: {}
 
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index 6bd305306f184797988115a5d6acb92e8e9178e7..168a4744eba6a34e00cb7bc0825818f322d1f512 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -84,6 +84,33 @@ def test_belongs_to_with_primary_key_joins_on_correct_column
     end
   end
 
+  def test_optional_relation_can_be_set_per_model
+    model1 = Class.new(ActiveRecord::Base) do
+      self.table_name = "accounts"
+      self.belongs_to_required_by_default = false
+
+      belongs_to :company
+
+      def self.name
+        "FirstModel"
+      end
+    end.new
+
+    model2 = Class.new(ActiveRecord::Base) do
+      self.table_name = "accounts"
+      self.belongs_to_required_by_default = true
+
+      belongs_to :company
+
+      def self.name
+        "SecondModel"
+      end
+    end.new
+
+    assert_predicate model1, :valid?
+    assert_not_predicate model2, :valid?
+  end
+
   def test_optional_relation
     original_value = ActiveRecord::Base.belongs_to_required_by_default
     ActiveRecord::Base.belongs_to_required_by_default = true
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 8eb9984872bf77dcaab67f973aa1cb2ae91d803f..d0872500fd91878a1a6715f59a02ac4faa526ae7 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -845,6 +845,26 @@ want to add this feature it will need to be turned on in an initializer.
 
     config.active_record.belongs_to_required_by_default = true
 
+The configuration is by default global for all your models, but you can
+override it on a per model basis. This should help you migrate all your models to have their
+associations required by default.
+
+    ```ruby
+      class Book < ApplicationRecord
+        # model is not yet ready to have its association required by default
+
+	self.belongs_to_required_by_default = false
+        belongs_to(:author)
+      end
+
+      class Car < ApplicationRecord
+        # model is ready to have its association required by default
+
+	self.belongs_to_required_by_default = true
+        belongs_to(:pilot)
+      end
+    ```
+
 #### Per-form CSRF Tokens
 
 Rails 5 now supports per-form CSRF tokens to mitigate against code-injection attacks with forms