Make `belongs_to_required_by_default` a class attribute:

- I'm hoping to get this change accepted even though this flag was
  introduced years ago in 6576f735

  My use case is the following:

  We were never enforcing belongs to association and we have a lot
  of models that implicitely declare an association as optional.
  We are now changing all our models to make associations required
  by default.
  Since we have a lot of models (more than 1000), I'd like to
  progressively enable them to use the `belongs_to_required_by_default`
  flag.

  The problem is that this flag is a mattr_accessor and doesn't to be
  set per model. We basically need to modify all our models (which
  could take years) before being able to modify the global flag.

  I'd like to change this flag to a class_attribute to solve the
  issue.

activerecord/CHANGELOG.md

+*   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*

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: {}
 

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

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