command_graph.rb

require 'rom'
require 'byebug'

rom = ROM.setup(:memory) do
  relation(:users)
  relation(:tasks)
  relation(:tags)

  commands(:users) do
    define(:create) do
      input Transproc(:accept_keys, [:name])
      result :one
    end
  end

  commands(:tasks) do
    define(:create) do
      input Transproc(:accept_keys, [:title, :user])

      def execute(tuples, user)
        super(tuples.map { |t| t.merge(user: user.fetch(:name)) })
      end
    end
  end

  commands(:tags) do
    define(:create) do
      input Transproc(:accept_keys, [:name, :task])

      def execute(tuples, task)
        super(tuples.map { |t| t.merge(task: task.fetch(:title)) })
      end
    end
  end

  mappers do
    define(:users) do
      model name: 'User'
      register_as :entity

      attribute :name

      combine :tasks, on: { name: :user } do
        model name: 'Task'
        attribute :title

        combine :tags, on: { title: :task } do
          model name: 'Tag'
          attribute :name
        end
      end
    end
  end
end

input = {
  user: {
    name: 'Jane',
    tasks: [
      {
        title: 'One',
        tags: [{ name: 'red' }]
      },
      {
        title: 'Two',
        tags: [{ name: 'green' }, { name: 'blue' }]
      }
    ]
  }
}

cmd = rom.command([
  { user: :users }, [
    :create, [
      :tasks, [:create, [:tags, [:create]]]
    ]
  ]
])

puts (cmd >> rom.mappers[:users].entity).call(input).to_a.inspect

output.txt

[#<User:0x007fc073f6caf8 @name="Jane", @tasks=[#<Task:0x007fc073f6d390 @title="One", @tags=[#<Tag:0x007fc073f6d5e8 @name="red">]>, #<Task:0x007fc073f6cd28 @title="Two", @tags=[#<Tag:0x007fc073f6d070 @name="green">, #<Tag:0x007fc073f6cf08 @name="blue">]>]>]