Same method for has_many and has_one associations

activerecord/lib/active_record/associations/builder/association.rb

@@ -65,5 +65,20 @@ def dependent_restrict_deprecation_warning
           ActiveSupport::Deprecation.warn msg
         end
       end
+
+      def define_restrict_dependency_method
+        name = self.name
+        mixin.redefine_method(dependency_method_name) do
+          # has_many or has_one associations
+          if send(name).respond_to?(:exists?) ? send(name).exists? : !send(name).nil?
+            if dependent_restrict_raises?
+              raise ActiveRecord::DeleteRestrictionError.new(name)
+            else
+              errors.add(:base, :restrict_dependent_destroy, :model => name)
+              return false
+            end
+          end
+        end
+      end
    end
 end

activerecord/lib/active_record/associations/builder/has_many.rb

@@ -53,20 +53,6 @@ def define_nullify_dependency_method
         end
       end
 
-      def define_restrict_dependency_method
-        name = self.name
-        mixin.redefine_method(dependency_method_name) do
-          if send(name).exists?
-            if dependent_restrict_raises?
-              raise ActiveRecord::DeleteRestrictionError.new(name)
-            else
-              errors.add(:base, :restrict_dependent_destroy, :model => name)
-              return false
-            end
-          end
-        end
-      end
-
       def dependency_method_name
         "has_many_dependent_for_#{name}"
       end

activerecord/lib/active_record/associations/builder/has_one.rb

@@ -40,10 +40,6 @@ def configure_dependency
         end
       end
 
-      def dependency_method_name
-        "has_one_dependent_#{options[:dependent]}_for_#{name}"
-      end
-
       def define_destroy_dependency_method
         name = self.name
         mixin.redefine_method(dependency_method_name) do
@@ -53,18 +49,8 @@ def define_destroy_dependency_method
       alias :define_delete_dependency_method :define_destroy_dependency_method
       alias :define_nullify_dependency_method :define_destroy_dependency_method
 
-      def define_restrict_dependency_method
-        name = self.name
-        mixin.redefine_method(dependency_method_name) do
-          unless send(name).nil?
-            if dependent_restrict_raises?
-              raise ActiveRecord::DeleteRestrictionError.new(name)
-            else
-              errors.add(:base, :restrict_dependent_destroy, :model => name)
-              return false
-            end
-          end
-        end
+      def dependency_method_name
+        "has_one_dependent_#{options[:dependent]}_for_#{name}"
       end
   end
 end