Solution to fix DeserializationErrors with old class references

.rake_tasks~

@@ -0,0 +1,7 @@
+build
+install
+install:local
+release[remote]
+rubocop
+rubocop:auto_correct
+spec

README.md

@@ -479,6 +479,22 @@ Cleaning up
 ===========
 You can invoke `rake jobs:clear` to delete all jobs in the queue.
 
+Upgrade old references
+===========================
+Maybe you get errors in your jobs like this.
+```
+ERROR -- : 2019-03-15T10:11:04+0000: [Worker(delayed_job host:xxx pid:776)] Job Delayed::PerformableMethod (id=1) FAILED permanently with Delayed::DeserializationError: Job failed to load: undefined class/module ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlDateTime
+```
+
+You have to search all conflictive references, find theirs new names and set to fix them.
+
+```ruby
+# config/initializers/delayed_job_config.rb
+Psych.old_class_references = {
+  'ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlDateTime' => 'ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString'
+}
+```
+
 Having problems?
 ================
 Good places to get help are:

lib/delayed/psych_ext.rb

@@ -12,10 +12,23 @@ def encode_with(coder)
 end
 
 module Psych
+  # allow set references to serialize old dependencies by news
+  class << self; attr_accessor :old_class_references end
+  @old_class_references = {}
+
   def self.load_dj(yaml)
     result = parse(yaml)
     result ? Delayed::PsychExt::ToRuby.create.accept(result) : result
   end
+
+  class ClassLoader
+  private
+
+    def find(klassname)
+      klassname = ::Psych.old_class_references[klassname] || klassname
+      @cache[klassname] ||= resolve(klassname)
+    end
+  end
 end
 
 module Delayed
@@ -97,6 +110,17 @@ def resolve_class(klass_name)
       rescue
         super
       end
+
+    private
+
+      def revive(klass, node)
+        if klass.is_a? String
+          klassname = ::Psych.old_class_references[klass] || klass
+          klass = Kernel.const_get(klassname) rescue klassname
+        end
+        s = register(node, klass.allocate)
+        init_with(s, revive_hash({}, node), node)
+      end
     end
   end
 end

spec/psych_ext_spec.rb

@@ -31,4 +31,24 @@
       expect(deserialized.instance_variable_get(:@check)).to eq(12)
     end
   end
+
+  context 'with a old reference' do
+    class NewKlass; end
+
+    let(:klass) { NewKlass.name }
+    let(:invalid_reference) { 'OldKlass' }
+    let(:invalid_yaml) { "--- !ruby/class '#{invalid_reference}'\n" }
+
+    it 'raise an error when is not refered' do
+      expect { YAML.load(invalid_yaml) }.to raise_error ArgumentError
+    end
+
+    it 'deserializes correctly when is refered' do
+      expect do
+        Psych.old_class_references = {invalid_reference => klass}
+        expect(YAML.load(invalid_yaml)).to be Kernel.const_get(klass)
+        Psych.old_class_references = {}
+      end.not_to raise_error
+    end
+  end
 end