Extending a module to models causes ArgumentError A copy of xx has been removed from module tree

How can I extend a module to a model class and use an instance of a
custom class as a class instance variable for model such that when next
request comes, custom class could be found and older instances are
deleted?

I created a class Samples. I also created a module LoadSamples where
in a method say load_samples, I create a new Samples instance and
assign it to a class instance variable. I extend LoadSamples to
ActiveRecord::Base and called load_samples from model User.

class Samples
...
end

module LoadSamples
  attr_accessor :samples

  def load_samples
    @samples ||= Samples.new
    ...
  end
end

The first page that loads, loads fine, but subsequent request gives: