5 files changed, 76 insertions, 2 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 21075d0636..c61407b927 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,44 @@
+*   Restore 4.0 behavior for using serialize attributes with `JSON` as coder.
+
+    With 4.1.x, `serialize` started returning a string when `JSON` was passed as
+    the second attribute. It will now return a hash as per previous versions.
+
+    Example:
+
+        class Post < ActiveRecord::Base
+          serialize :comment, JSON
+        end
+
+        class Comment
+          include ActiveModel::Model
+          attr_accessor :category, :text
+        end
+
+        post = Post.create!
+        post.comment = Comment.new(category: "Animals", text: "This is a comment about squirrels.")
+        post.save!
+
+        # 4.0
+        post.comment # => {"category"=>"Animals", "text"=>"This is a comment about squirrels."}
+
+        # 4.1 before
+        post.comment # => "#<Comment:0x007f80ab48ff98>"
+
+        # 4.1 after
+        post.comment # => {"category"=>"Animals", "text"=>"This is a comment about squirrels."}
+
+    When using `JSON` as the coder in `serialize`, Active Record will use the
+    new `ActiveRecord::Coders::JSON` coder which delegates its `dump/load` to
+    `ActiveSupport::JSON.encode/decode`. This ensures special objects are dumped
+    correctly using the `#as_json` hook.
+
+    To keep the previous behaviour, supply a custom coder instead
+    ([example](https://gist.github.com/jenncoop/8c4142bbe59da77daa63)).
+
+    Fixes #15594.
+
+    *Jenn Cooper*
+
 *   Add a `:required` option to singular associations, providing a nicer
     API for presence validations on associations.
 
diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index 17b00bbaea..9028970a3d 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -97,6 +97,7 @@ module ActiveRecord
 
   module Coders
     autoload :YAMLColumn, 'active_record/coders/yaml_column'
+    autoload :JSON, 'active_record/coders/json'
   end
 
   module AttributeMethods
diff --git a/activerecord/lib/active_record/attribute_methods/serialization.rb b/activerecord/lib/active_record/attribute_methods/serialization.rb
index 734d94865a..264ce2bdfa 100644
--- a/activerecord/lib/active_record/attribute_methods/serialization.rb
+++ b/activerecord/lib/active_record/attribute_methods/serialization.rb
@@ -37,7 +37,12 @@ module ActiveRecord
         #     serialize :preferences, Hash
         #   end
         def serialize(attr_name, class_name_or_coder = Object)
-          coder = if [:load, :dump].all? { |x| class_name_or_coder.respond_to?(x) }
+          # When ::JSON is used, force it to go through the Active Support JSON encoder
+          # to ensure special objects (e.g. Active Record models) are dumped correctly
+          # using the #as_json hook.
+          coder = if class_name_or_coder == ::JSON
+                    Coders::JSON
+                  elsif [:load, :dump].all? { |x| class_name_or_coder.respond_to?(x) }
                     class_name_or_coder
                   else
                     Coders::YAMLColumn.new(class_name_or_coder)
diff --git a/activerecord/lib/active_record/coders/json.rb b/activerecord/lib/active_record/coders/json.rb
new file mode 100644
index 0000000000..0f60b6dab2
--- /dev/null
+++ b/activerecord/lib/active_record/coders/json.rb
@@ -0,0 +1,13 @@
+module ActiveRecord
+  module Coders # :nodoc:
+    class JSON # :nodoc:
+      def self.dump(obj)
+        ActiveSupport::JSON.encode(obj)
+      end
+
+      def self.load(json)
+        ActiveSupport::JSON.decode(json)
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/serialized_attribute_test.rb b/activerecord/test/cases/serialized_attribute_test.rb
index 186a1a2ade..a8f667c732 100644
--- a/activerecord/test/cases/serialized_attribute_test.rb
+++ b/activerecord/test/cases/serialized_attribute_test.rb
@@ -3,10 +3,11 @@ require 'models/topic'
 require 'models/reply'
 require 'models/person'
 require 'models/traffic_light'
+require 'models/post'
 require 'bcrypt'
 
 class SerializedAttributeTest < ActiveRecord::TestCase
-  fixtures :topics
+  fixtures :topics, :posts
 
   MyObject = Struct.new :attribute1, :attribute2
 
@@ -67,6 +68,19 @@ class SerializedAttributeTest < ActiveRecord::TestCase
     assert_equal(orig.content, clone.content)
   end
 
+  def test_serialized_json_attribute_returns_unserialized_value
+    Topic.serialize :content, JSON
+    my_post = posts(:welcome)
+
+    t = Topic.new(content: my_post)
+    t.save!
+    t.reload
+
+    assert_instance_of(Hash, t.content)
+    assert_equal(my_post.id, t.content["id"])
+    assert_equal(my_post.title, t.content["title"])
+  end
+
   def test_serialized_attribute_declared_in_subclass
     hash = { 'important1' => 'value1', 'important2' => 'value2' }
     important_topic = ImportantTopic.create("important" => hash)