Test for Lotus::Model associations

associations_test.rb

require 'bundler/setup'
require 'lotus/model'
require 'lotus/model/migrator'
require 'pg'

class Bicycle
  include Lotus::Entity
  attributes :parts
end

class BicycleRepository
  include Lotus::Repository
end

class Part
  include Lotus::Entity
  attributes :bicycle_id, :name
end

class PartRepository
  include Lotus::Repository
end

Lotus::Model.configure do
  adapter type: :sql, uri: 'postgres://localhost/associations_test'

  mapping do
    collection :bicycles do
      entity     Bicycle
      repository BicycleRepository

      attribute   :id,    Integer
      association :parts, [Part], collection: :parts, foreign_key: :user_id
    end

    collection :parts do
      entity     Part
      repository PartRepository

      attribute :id,         Integer
      attribute :bicycle_id, Integer
      attribute :name,       String
    end
  end
end.load!

Lotus::Model::Migrator.prepare

Lotus::Model.migration do
  change do
    create_table :bicycles do
      primary_key :id
    end

    create_table :parts do
      primary_key :id
      foreign_key :bicycle_id, :bicycles, on_delete: :cascade, null: false

      column :name, String
    end
  end
end.apply(::Sequel::DATABASES.first, :up)

b = Bicycle.new
b.parts << Part.new(name: 'wheel')

b = BicycleRepository.create(b)
puts b.inspect

First attempt

bundle exec ruby association_test.rb
association_test.rb:65:in `<main>': undefined method `<<' for nil:NilClass (NoMethodError)

Second attempt

Then I patched Bicycle like this:

class Bicycle
  include Lotus::Entity
  attributes :parts

  def parts
    @parts || []
  end
end

bundle exec ruby association_test.rb
#<Bicycle:0x007f84aa161e40 @id=1 @parts=[]>
[]

What's the intention here? Is to save all Part when save Bicycle with BicycleRepository.create(bicycle)?