Deprecate serialized_attributes without replacement

We've stopped using it internally, in favor of polymorphism. So should
you!

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Deprecate `serialized_attributes` without replacement. You can access its
+    behavior by going through the column's type object.
+
+    *Sean Griffin*
+
 *   Correctly extract IPv6 addresses from `DATABASE_URI`: the square brackets
     are part of the URI structure, not the actual host.
 

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -3,20 +3,7 @@ module ActiveRecord
     module Serialization
       extend ActiveSupport::Concern
 
-      included do
-        # Returns a hash of all the attributes that have been specified for
-        # serialization as keys and their class restriction as values.
-        class_attribute :serialized_attributes, instance_accessor: false
-        self.serialized_attributes = {}
-      end
-
       module ClassMethods
-        ##
-        # :method: serialized_attributes
-        #
-        # Returns a hash of all the attributes that have been specified for
-        # serialization as keys and their class restriction as values.
-
         # If you have an attribute that needs to be saved to the database as an
         # object, and retrieved as the same object, then specify the name of that
         # attribute using this method and it will be handled automatically. The
@@ -59,10 +46,24 @@ module ActiveRecord
           decorate_attribute_type(attr_name, :serialize) do |type|
             Type::Serialized.new(type, coder)
           end
+        end
 
-          # merge new serialized attribute and create new hash to ensure that each class in inheritance hierarchy
-          # has its own hash of own serialized attributes
-          self.serialized_attributes = serialized_attributes.merge(attr_name.to_s => coder)
+        def serialized_attributes
+          ActiveSupport::Deprecation.warn(<<-WARNING.strip_heredoc)
+            `serialized_attributes` is deprecated, and will be removed in Rails 5.0.
+            If you need to access the serialization behavior, you can do:
+
+              #{self.class.name}.column_for_attribute('foo').type_cast_for_database(value)
+
+            or
+
+              #{self.class.name}.column_for_attribute('foo').type_cast_from_database(value)
+          WARNING
+          @serialized_attributes ||= Hash[
+            columns.select { |t| t.cast_type.is_a?(Type::Serialized) }.map { |c|
+              [c.name, c.cast_type.coder]
+            }
+          ]
         end
       end
     end

activerecord/test/cases/serialized_attribute_test.rb

@@ -22,7 +22,9 @@ class SerializedAttributeTest < ActiveRecord::TestCase
   end
 
   def test_list_of_serialized_attributes
-    assert_equal %w(content), Topic.serialized_attributes.keys
+    assert_deprecated do
+      assert_equal %w(content), Topic.serialized_attributes.keys
+    end
   end
 
   def test_serialized_attribute
@@ -207,7 +209,7 @@ class SerializedAttributeTest < ActiveRecord::TestCase
     t = Topic.create(content: "first")
     assert_equal("first", t.content)
 
-    t.update_column(:content, Topic.serialized_attributes["content"].dump("second"))
+    t.update_column(:content, Topic.type_for_attribute('content').type_cast_for_database("second"))
     assert_equal("second", t.content)
   end