level_0.3_repository.rb

module InterestRepository
  extend self
  
  def my_index
    Interest.where(fancy: 'query')
            .and(more: 'fancy_stuff')
            .order('wicked')
            .joins('allthethings')
  end
end

class Interest < ActiveRecord::Base
  validates :account_id, :amount, :rate, presence: true
  belongs_to :account
end