update_solargraph.rb

namespace :solargraph do
  task update_model_definitions: :environment do
    if Rails.env.development?
      Rails.application.eager_load!
      def_file = Rails.root.join('config', 'model_definitions.rb')
      File.open(def_file, 'w') do |file|
        ApplicationRecord.descendants.each do |model|
          file << "class #{model.name}\n"
          klass = model.name.constantize
          klass.attribute_names.each do |name|
            file << "#   @!attribute [rw] #{name}\n"
            file << "#     @return [#{ruby_type[klass.type_for_attribute(name).type]}]\n"
          end
          file << "end\n"
        end
      end
    end
  end
end

def ruby_type
  {
    array:       'Array',
    boolean:     'Boolean',
    date:        'Date',
    datetime:    'DateTime',
    decimal:     'Decimal',
    float:       'Float',
    integer:     'Integer',
    string:      'String',
    text:        'String',
    time:        'Time',
  }
end

I had to do
Zeitwerk::Loader.eager_load_all if Zeitwerk instead of Rails.application.eager_load! to get it working on Rails 6 app