Serializing postgres arrays (#1018)

This PR addresses #1015

Starting from Rails version 5.0.2 the default serializer of PostgreSQL columns returns an ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Array object instead of a string. This new object is not suitable for JSON encoding and breaks versioning of any array fields backed by Postgres.

Whenever a PostgreSQL array is used, instead of asking Active Record for a serializer we introduce our own, which simply returns the underlying array without any modifications.

lib/paper_trail/attribute_serializers/attribute_serializer_factory.rb

@@ -0,0 +1,24 @@
+require "paper_trail/type_serializers/postgres_array_serializer"
+
+module PaperTrail
+  module AttributeSerializers
+    # Values returned by some Active Record serializers are
+    # not suited for writing JSON to a text column. This factory
+    # replaces certain default Active Record serializers
+    # with custom PaperTrail ones.
+    module AttributeSerializerFactory
+      def self.for(klass, attr)
+        active_record_serializer = klass.type_for_attribute(attr)
+        case active_record_serializer
+        when ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Array then
+          TypeSerializers::PostgresArraySerializer.new(
+            active_record_serializer.subtype,
+            active_record_serializer.delimiter
+          )
+        else
+          active_record_serializer
+        end
+      end
+    end
+  end
+end

lib/paper_trail/attribute_serializers/cast_attribute_serializer.rb

@@ -1,3 +1,5 @@
+require "paper_trail/attribute_serializers/attribute_serializer_factory"
+
 module PaperTrail
   # :nodoc:
   module AttributeSerializers
@@ -32,7 +34,7 @@ module PaperTrail
       # This implementation uses AR 5's `serialize` and `deserialize`.
       class CastAttributeSerializer
         def serialize(attr, val)
-          @klass.type_for_attribute(attr).serialize(val)
+          AttributeSerializerFactory.for(@klass, attr).serialize(val)
         end
 
         def deserialize(attr, val)
@@ -40,7 +42,7 @@ module PaperTrail
             # Because PT 4 used to save the string version of enums to `object_changes`
             val
           else
-            @klass.type_for_attribute(attr).deserialize(val)
+            AttributeSerializerFactory.for(@klass, attr).deserialize(val)
           end
         end
       end

lib/paper_trail/type_serializers/postgres_array_serializer.rb

@@ -0,0 +1,47 @@
+module PaperTrail
+  module TypeSerializers
+    # Provides an alternative method of serialization
+    # and deserialization of PostgreSQL array columns.
+    class PostgresArraySerializer
+      def initialize(subtype, delimiter)
+        @subtype = subtype
+        @delimiter = delimiter
+      end
+
+      def serialize(array)
+        return serialize_with_ar(array) if active_record_pre_502?
+        array
+      end
+
+      def deserialize(array)
+        return deserialize_with_ar(array) if active_record_pre_502?
+
+        case array
+        # Needed for legacy reasons. If serialized array is a string
+        # then it was serialized with Rails < 5.0.2.
+        when ::String then deserialize_with_ar(array)
+        else array
+        end
+      end
+
+      private
+
+      def active_record_pre_502?
+        ::ActiveRecord::VERSION::MAJOR < 5 ||
+          (::ActiveRecord::VERSION::MINOR.zero? && ::ActiveRecord::VERSION::TINY < 2)
+      end
+
+      def serialize_with_ar(array)
+        ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Array.
+          new(@subtype, @delimiter).
+          serialize(array)
+      end
+
+      def deserialize_with_ar(array)
+        ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Array.
+          new(@subtype, @delimiter).
+          deserialize(array)
+      end
+    end
+  end
+end

spec/dummy_app/app/models/user.rb

@@ -0,0 +1,3 @@
+class PostgresUser < ActiveRecord::Base
+  has_paper_trail
+end

spec/dummy_app/db/migrate/20110208155312_set_up_test_tables.rb

@@ -61,6 +61,15 @@ class SetUpTestTables < (
       t.timestamps null: true
     end
 
+    if ENV["DB"] == "postgres"
+      create_table :postgres_users, force: true do |t|
+        t.string     :name
+        t.integer    :post_ids,    array: true
+        t.datetime   :login_times, array: true
+        t.timestamps null: true
+      end
+    end
+
     create_table :versions, versions_table_options do |t|
       t.string   :item_type, item_type_options
       t.integer  :item_id,   null: false

spec/paper_trail/attribute_serializers/object_attribute_spec.rb

@@ -0,0 +1,43 @@
+require "spec_helper"
+
+module PaperTrail
+  module AttributeSerializers
+    ::RSpec.describe ObjectAttribute do
+      if ENV["DB"] == "postgres" && ::ActiveRecord::VERSION::MAJOR >= 5
+        describe "postgres-specific column types" do
+          describe "#serialize" do
+            it "serializes a postgres array into a plain array" do
+              attrs = { "post_ids" => [1, 2, 3] }
+              described_class.new(PostgresUser).serialize(attrs)
+              expect(attrs["post_ids"]).to eq [1, 2, 3]
+            end
+          end
+
+          describe "#deserialize" do
+            it "deserializes a plain array correctly" do
+              attrs = { "post_ids" => [1, 2, 3] }
+              described_class.new(PostgresUser).deserialize(attrs)
+              expect(attrs["post_ids"]).to eq [1, 2, 3]
+            end
+
+            it "deserializes an array serialized with Rails <= 5.0.1 correctly" do
+              attrs = { "post_ids" => "{1,2,3}" }
+              described_class.new(PostgresUser).deserialize(attrs)
+              expect(attrs["post_ids"]).to eq [1, 2, 3]
+            end
+
+            it "deserializes an array of time objects correctly" do
+              date1 = 1.day.ago
+              date2 = 2.days.ago
+              date3 = 3.days.ago
+              attrs = { "post_ids" => [date1, date2, date3] }
+              described_class.new(PostgresUser).serialize(attrs)
+              described_class.new(PostgresUser).deserialize(attrs)
+              expect(attrs["post_ids"]).to eq [date1, date2, date3]
+            end
+          end
+        end
+      end
+    end
+  end
+end