level_1_repository.rb

module InterestRepository
  extend self

  def my_index
    InterestRecord.includes(:account).order(percentage: :desc).all.map do |interest_data|
      Interest.new(interest_data.attributes_with_associations)
    end
  end

  def fetch(id)
    Interest.new(InterestRecord.includes(:account).find(id)).attributes_with_associations
  end

  def save(interest)
    return false unless interest.valid?

    data = interest.id ? InterestRecord.find(interest.id) : InterestRecord.new

    data.assign_attributes(interest.attributes)
    data.save
  end
end

class Interest
  include ActiveModel::Model

  ATTRIBUTES_FOR_DATA = %i[account_id amount rate].freeze

  attr_accessor(*ATTRIBUTES_FOR_DATA, :account, :id, :created_at, :updated_at)

  validates :account_id, :amount, :rate, presence: true

  def attributes
    ATTRIBUTES_FOR_DATA.index_with do |attribute|
      send(attribute)
    end
  end
end

class InterestRecord < ActiveRecord::Base
  self.table_name = 'interests'

  belongs_to :account

  def attributes_with_associations
    attributes.merge('account' => account)
  end
end