2 files changed, 12 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index 64b2311911..4a4f88bb94 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -21,6 +21,10 @@ module ActiveRecord
         self.target = record
       end
 
+      def default(record)
+        writer(record) if reader.nil?
+      end
+
       def reset
         super
         @updated = false
diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index a1609ab0fb..50a1c39ccf 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 # :nodoc:
     end
 
     def self.valid_options(options)
-      super + [:polymorphic, :touch, :counter_cache, :optional]
+      super + [:polymorphic, :touch, :counter_cache, :optional, :default]
     end
 
     def self.valid_dependent_options
@@ -16,6 +16,7 @@ module ActiveRecord::Associations::Builder # :nodoc:
       super
       add_counter_cache_callbacks(model, reflection) if reflection.options[:counter_cache]
       add_touch_callbacks(model, reflection)         if reflection.options[:touch]
+      add_default_callbacks(model, reflection)       if reflection.options[:default]
     end
 
     def self.define_accessors(mixin, reflection)
@@ -118,6 +119,12 @@ module ActiveRecord::Associations::Builder # :nodoc:
       model.after_destroy callback.(:changes_to_save)
     end
 
+    def self.add_default_callbacks(model, reflection)
+      model.before_validation lambda { |o|
+        o.association(reflection.name).default o.instance_exec(&reflection.options[:default])
+      }
+    end
+
     def self.add_destroy_callbacks(model, reflection)
       model.after_destroy lambda { |o| o.association(reflection.name).handle_dependency }
     end