processable.rb

require 'rails_helper'

RSpec.describe Processable do
  # ...
  
  describe '#mark_as_processed!' do
    it 'updates the processed column to true' do
      subject
        .expects(:update_column)
        .with(:processed, true)
        .once
        .returns(true)

      subject.mark_as_processed!
    end
  end
  
  # ...
end