From a0751d27c52185b86370baf6a565e757092646de Mon Sep 17 00:00:00 2001
From: Kir Shatrov <shatrov@me.com>
Date: Wed, 21 Jun 2017 13:58:07 -0400
Subject: [PATCH] Do not let use `serialize` on native JSON/array column

---
 .../attribute_methods/serialization.rb        | 24 +++++++++++++++++++
 .../cases/adapters/postgresql/array_test.rb   |  9 +++++++
 .../cases/adapters/postgresql/json_test.rb    |  9 +++++++
 3 files changed, 42 insertions(+)

diff --git a/activerecord/lib/active_record/attribute_methods/serialization.rb b/activerecord/lib/active_record/attribute_methods/serialization.rb
index 6ed45d8737..acd47629dd 100644
--- a/activerecord/lib/active_record/attribute_methods/serialization.rb
+++ b/activerecord/lib/active_record/attribute_methods/serialization.rb
@@ -5,6 +5,16 @@ module ActiveRecord
     module Serialization
       extend ActiveSupport::Concern
 
+      class ColumnNotSerializableError < StandardError
+        def initialize(name, type)
+          super <<-EOS.strip_heredoc
+            Column `#{name}` of type #{type.class} does not support `serialize` feature.
+            Usually it means that you are trying to use `serialize`
+            on a column that already implements serialization natively.
+          EOS
+        end
+      end
+
       module ClassMethods
         # 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
@@ -60,9 +70,23 @@ module ActiveRecord
           end
 
           decorate_attribute_type(attr_name, :serialize) do |type|
+            if type_incompatible_with_serialize?(type)
+              raise ColumnNotSerializableError.new(attr_name, type)
+            end
+
             Type::Serialized.new(type, coder)
           end
         end
+
+        private
+
+          def type_incompatible_with_serialize?(type)
+            type.is_a?(ActiveRecord::Type::Json) ||
+            (
+              defined?(ActiveRecord::ConnectionAdapters::PostgreSQL) &&
+              type.is_a?(ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Array)
+            )
+          end
       end
     end
   end
diff --git a/activerecord/test/cases/adapters/postgresql/array_test.rb b/activerecord/test/cases/adapters/postgresql/array_test.rb
index 8507dbb463..08b17f37e2 100644
--- a/activerecord/test/cases/adapters/postgresql/array_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/array_test.rb
@@ -47,6 +47,15 @@ class PostgresqlArrayTest < ActiveRecord::PostgreSQLTestCase
     assert ratings_column.array?
   end
 
+  def test_not_compatible_with_serialize
+    new_klass = Class.new(PgArray) do
+      serialize :tags, Array
+    end
+    assert_raises(ActiveRecord::AttributeMethods::Serialization::ColumnNotSerializableError) do
+      new_klass.new
+    end
+  end
+
   def test_default
     @connection.add_column "pg_arrays", "score", :integer, array: true, default: [4, 4, 2]
     PgArray.reset_column_information
diff --git a/activerecord/test/cases/adapters/postgresql/json_test.rb b/activerecord/test/cases/adapters/postgresql/json_test.rb
index aa5e03df41..79dcfe110c 100644
--- a/activerecord/test/cases/adapters/postgresql/json_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/json_test.rb
@@ -33,6 +33,15 @@ module PostgresqlJSONSharedTestCases
     x.reload
     assert_equal ["foo" => "bar"], x.objects
   end
+
+  def test_not_compatible_with_serialize_macro
+    new_klass = Class.new(klass) do
+      serialize :payload, JSON
+    end
+    assert_raises(ActiveRecord::AttributeMethods::Serialization::ColumnNotSerializableError) do
+      new_klass.new
+    end
+  end
 end
 
 class PostgresqlJSONTest < ActiveRecord::PostgreSQLTestCase