test_rom3.rb

require 'rom-repository'

rom = ROM.container(:sql, 'sqlite::memory') do |config|
  config.default.create_table(:users) do
    primary_key :id
    column :name, String, null: false
    column :email, String, null: false
  end

  config.default.create_table(:tasks) do
    primary_key :id
    foreign_key :user_id, :users
    column :title, String, null: false
  end

  config.default.create_table(:task_details) do
    primary_key :id
    foreign_key :task_id, :tasks
    column :content, String, null: false
  end

  config.relation(:users) do
    schema(infer: true) do
      associations do
        has_many :tasks
      end
    end
  end

  config.relation(:tasks) do
    schema(infer: true) do
      associations do
        belongs_to :user
        has_many :task_details
      end
    end
  end

  config.relation(:task_details) do
    schema(infer: true) do
      associations do
        belongs_to :task
      end
    end
  end
end

class UserRepo < ROM::Repository[:users]
  relations :tasks
  relations :task_details

  def create_with_tasks(user)
    command(:create, aggregate(tasks: :task_details)).call(user)
  end
end

class TaskRepo < ROM::Repository[:tasks]
  relations :task_details
end

user_repo = UserRepo.new(rom)

require 'pry'
binding.pry

user_repo.create_with_tasks(
  name: 'Jane',
  email: 'jane@doe.org',
  tasks: [{
    title: 'Task 1',
    task_details: {
      content: 'Content 1'
    }
  }, {
    title: 'Task 2',
    task_details: {
      content: 'Content 2'
    }
  }]
)