Rails model does not load properly - ruby-on-rails

Question

I updated my Rails application to a newer version of Rails, and now one of my models is unusable, as if it was loaded empty, as if Rails was not aware of its structure and how to populate it.
This model is Task and is based on Single Table Inheritance:
class Activity < ActiveRecord::Base
(...)
end
class Task < Activity
(...)
end
class Event < Activity
(...)
end
class Phase < Activity
(...)
end
Here is what is expected when the model is loaded properly and everything works well (same model because of STI):
Activity.inspect => Activity(id: integer, type: string, project_id: integer, author_id: integer, subject: text, description: text, (...), created_at: datetime, updated_at: datetime)
Task.inspect => Task(id: integer, type: string, project_id: integer, author_id: integer, subject: text, description: text, (...), created_at: datetime, updated_at: datetime)
Event.inspect => Event(same model as above ...)
(...)
However, since the update, Rails is unable to use the Task model at all, neither for reading a record from the database or creating a new one. Other related models Event and Phase works as expected.
When I debug it in production mode by inspecting the Task class and Task.first from the Task controller # show I get:
Task.inspect => Task()
Task.first.inspect => #<Task >
Task.first.nil? => false
This model appears as if it was void. Others models are loaded perfectly.
The application does not fail with an error when Rails tries to load the record, but it fails later when the controller tries to access any field, because the model is empty.
Some important observations :
It only fails in production mode, but works well in development mode, or it fails only when config.cache_classes = true but works well otherwise.
It works well in the Rails console.
task.rb model file gets loaded even in the failing context. Model files structure is flat, all models are in app/models/
It used to work well before updating from Ruby 1.9.3 and Rails 3.1 to Ruby 2.3.8 and Rails 3.2.22.

Answer 1

This is probably a duplicate of https://stackoverflow.com/a/58867450/1388302
Something else is defining Task - maybe to extend it. Don't do that. Use prepend. Otherwise the autoloader finds that unintentional definition and won't load the one you intend.
Or Task is now also defined by something you're including. It might be interesting to ask about Task.superclass.
This probably came up because Rails 6 changed the autoloader.

Answer 2

Something looks buggy with respect to Rails 3.2.22 schema loading and STI.
The best workaround I found so far is to force schema reloading for this model using :
Task.connection.schema_cache.clear!
Task.reset_column_information
But then Rails looses track of STI and will miss adding the "type" field to the newly created records. So to force this fields, in every controller required, I added :
#task = (...).becomes(Task)
This feels hacky, but I could not find any better way. Maybe this tip will help someone else. In my case it was not an option to upgrade Rails further or to refactor all the code to get rid of STI.