gistfile1.rb

require 'rom-sql'

setup = ROM.setup('postgres://localhost/rom')

setup.default.adapter.connection.drop_table?(:users)
setup.default.adapter.connection.drop_table?(:groups)

setup.default.adapter.connection.create_table :groups do
  Integer :id
  String :name, null: false
  primary_key [:id]
end

setup.default.adapter.connection.create_table :users do
  Integer :id
  String :name, null: false
  Integer :age, null: false
  primary_key [:id]
  foreign_key :group, :groups
end

setup.relation(:groups) do
  one_to_many :users, key: :group

  def with_users
    select(:id, :name)
      .prefix(:group)
      .association_join(:users, select: [:id, :name, :age])
  end
end

setup.commands(:users) { define(:create) }
setup.commands(:groups) { define(:create) }

setup.mappers do
  define(:groups, prefix: :group, inherit_header: false) do
    model name: 'Group'

    attribute :id
    attribute :name

    group :users, prefix: false do
      model name: 'User'

      attribute :id
      attribute :name
      attribute :age
    end
  end
end

rom = setup.finalize

rom.command(:groups).try{ create(id: 1, name: "First Group")}
rom.command(:users).try{ create(id: 1, name: "Edson Michaque", age: 128, group: 1)}
rom.command(:users).try{ create(id: 2, name: "Alberto Michaque", age: 256, group: 1)}

puts rom.read(:groups).with_users.to_a.inspect