From 307ec3db0fe26cbd1811d34a27e6637726ce23ce Mon Sep 17 00:00:00 2001
From: Sean Griffin <sean@thoughtbot.com>
Date: Sat, 27 Dec 2014 19:46:36 -0700
Subject: [PATCH] Provide a better error message for unsupported classes in
 `serialize`

We only support classes which provide a no-args constructor to use as a
default value. We can provide a more helpful error message if we catch
this when `serialize` is called, rather than letting it error when you
try to assign the attribute.

Fixes #18224
---
 activerecord/CHANGELOG.md                            |  7 +++++++
 activerecord/lib/active_record/coders/yaml_column.rb | 11 +++++++++++
 activerecord/test/cases/serialized_attribute_test.rb |  6 ++++++
 3 files changed, 24 insertions(+)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 5dd5c53e9c..4a954c2cc6 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Provide a more helpful error message when an unsupported class is passed to
+    `serialize`
+
+    Fixes #18224
+
+    *Sean Griffin*
+
 *   Add bigint primary key support for MySQL.
 
     Example:
diff --git a/activerecord/lib/active_record/coders/yaml_column.rb b/activerecord/lib/active_record/coders/yaml_column.rb
index d3d7396c91..9ea22ed798 100644
--- a/activerecord/lib/active_record/coders/yaml_column.rb
+++ b/activerecord/lib/active_record/coders/yaml_column.rb
@@ -8,6 +8,7 @@ class YAMLColumn # :nodoc:
 
       def initialize(object_class = Object)
         @object_class = object_class
+        check_arity_of_constructor
       end
 
       def dump(obj)
@@ -33,6 +34,16 @@ def load(yaml)
 
         obj
       end
+
+      private
+
+      def check_arity_of_constructor
+        begin
+          load(nil)
+        rescue ArgumentError
+          raise ArgumentError, "Cannot serialize #{object_class}. Classes passed to `serialize` must have a 0 argument constructor."
+        end
+      end
     end
   end
 end
diff --git a/activerecord/test/cases/serialized_attribute_test.rb b/activerecord/test/cases/serialized_attribute_test.rb
index c8441201ca..c261a5b1c0 100644
--- a/activerecord/test/cases/serialized_attribute_test.rb
+++ b/activerecord/test/cases/serialized_attribute_test.rb
@@ -264,4 +264,10 @@ def test_nil_is_not_changed_when_serialized_with_a_class
 
     assert_not topic.content_changed?
   end
+
+  def test_classes_without_no_arg_constructors_are_not_supported
+    assert_raises(ArgumentError) do
+      Topic.serialize(:content, Regexp)
+    end
+  end
 end