Require `belongs_to` by default.

Deprecate `required` option in favor of `optional` for belongs_to.

5 files changed, 35 insertions, 9 deletions

diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 499b00a815..0b33ee881b 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1520,10 +1520,16 @@ module ActiveRecord
       #   object that is the inverse of this <tt>belongs_to</tt> association. Does not work in
       #   combination with the <tt>:polymorphic</tt> options.
       #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
+      # [:optional]
+      #   When set to +true+, the association will not have its presence validated.
       # [:required]
       #   When set to +true+, the association will also have its presence validated.
       #   This will validate the association itself, not the id. You can use
       #   +:inverse_of+ to avoid an extra query during validation.
+      #   NOTE: <tt>required</tt> is set to <tt>true</tt> by default and is deprecated. If
+      #   you don't want to have association presence validated, use <tt>optional: true</tt>.
+      #
+      #
       #
       # Option examples:
       #   belongs_to :firm, foreign_key: "client_of"
@@ -1536,7 +1542,7 @@ module ActiveRecord
       #   belongs_to :post, counter_cache: true
       #   belongs_to :comment, touch: true
       #   belongs_to :company, touch: :employees_last_updated_at
-      #   belongs_to :user, required: true
+      #   belongs_to :user, optional: true
       def belongs_to(name, scope = nil, options = {})
         reflection = Builder::BelongsTo.build(self, name, scope, options)
         Reflection.add_reflection self, name, reflection
diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index d0ad57f9c6..ec135d49b7 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -5,7 +5,7 @@ module ActiveRecord::Associations::Builder
     end
 
     def self.valid_options(options)
-      super + [:foreign_type, :polymorphic, :touch, :counter_cache]
+      super + [:foreign_type, :polymorphic, :touch, :counter_cache, :optional]
     end
 
     def self.valid_dependent_options
@@ -110,5 +110,23 @@ module ActiveRecord::Associations::Builder
       name = reflection.name
       model.after_destroy lambda { |o| o.association(name).handle_dependency }
     end
+
+    def self.define_validations(model, reflection)
+      if reflection.options.key?(:required)
+        reflection.options[:optional] = !reflection.options.delete(:required)
+      end
+
+      if reflection.options[:optional].nil?
+        required = model.belongs_to_required_by_default
+      else
+        required = !reflection.options[:optional]
+      end
+
+      super
+
+      if required
+        model.validates_presence_of reflection.name, message: :required
+      end
+    end
   end
 end
diff --git a/activerecord/lib/active_record/associations/builder/has_one.rb b/activerecord/lib/active_record/associations/builder/has_one.rb
index 64e9e6b334..a272d3c781 100644
--- a/activerecord/lib/active_record/associations/builder/has_one.rb
+++ b/activerecord/lib/active_record/associations/builder/has_one.rb
@@ -17,5 +17,12 @@ module ActiveRecord::Associations::Builder
     def self.add_destroy_callbacks(model, reflection)
       super unless reflection.options[:through]
     end
+
+    def self.define_validations(model, reflection)
+      super
+      if reflection.options[:required]
+        model.validates_presence_of reflection.name, message: :required
+      end
+    end
   end
 end
diff --git a/activerecord/lib/active_record/associations/builder/singular_association.rb b/activerecord/lib/active_record/associations/builder/singular_association.rb
index f6274c027e..42542f188e 100644
--- a/activerecord/lib/active_record/associations/builder/singular_association.rb
+++ b/activerecord/lib/active_record/associations/builder/singular_association.rb
@@ -27,12 +27,5 @@ module ActiveRecord::Associations::Builder
         end
       CODE
     end
-
-    def self.define_validations(model, reflection)
-      super
-      if reflection.options[:required]
-        model.validates_presence_of reflection.name, message: :required
-      end
-    end
   end
 end
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 1244bd6195..de48681746 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -87,6 +87,8 @@ module ActiveRecord
 
       mattr_accessor :maintain_test_schema, instance_accessor: false
 
+      mattr_accessor :belongs_to_required_by_default, instance_accessor: false
+
       class_attribute :default_connection_handler, instance_writer: false
 
       def self.connection_handler