aggregate_parent.rb

require 'rom-sql'
require 'rom-repository'

rom = ROM.container(:sql, 'sqlite::memory') do |c|
  c.gateways[:default].create_table :events do
    primary_key :id
    column :event_no, Integer
  end

  c.gateways[:default].create_table :orders do
    primary_key :id
    column :event_id, Integer
    column :name, String
  end

  c.gateways[:default].use_logger(Logger.new($stdout))

  c.relation(:events) do
    schema(infer: true) do
      associations do
        has_many :orders
      end
    end
  end

  c.relation(:orders) do
    schema(infer: true) do
      associations do
        belongs_to :event
      end
    end
  end
end

class OrderRepo < Class.new(ROM::Repository::Root)[:orders]
  relations :events
  def with_event
    aggregate(:event).to_a
  end
end

rom.relations[:events].insert event_no: 1
rom.relations[:events].insert event_no: 2

rom.relations[:orders].insert(name: 'Order #1', event_id: 1)
rom.relations[:orders].insert(name: 'Order #2', event_id: 1)


repo = OrderRepo.new(rom)

require "pry"
binding.pry