rom_nested_relation_maper.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required.'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rom'
  gem 'rom-sql'
  gem 'rom-repository'
  gem 'sqlite3'
  gem 'rspec'
  gem 'pry'
end

require 'rspec/autorun'

class User < Dry::Struct
  attribute :name, Dry::Types['strict.string']
end

class Post < Dry::Struct
  attribute :title, Dry::Types['strict.string']
  attribute :user, User
end

class UsersMapper < ROM::Mapper
  relation :users
  register_as :entity

  attribute :name, from: :nname

  model User
end

describe ROM::Repository do
  let(:rom) { ROM.container(configuration) }
  let(:configuration) do
    ROM::Configuration.new(:sql, 'sqlite::memory').tap do |config|
      config.default.create_table(:users) do
        primary_key :id
        column :nname, String, null: false
      end

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

      config.relation(:users) do
        schema(infer: true)

        view :as_entity do
          schema { rename(nname: :name) }
          relation { as(:entity) }
        end
      end

      config.register_mapper(UsersMapper)
    end
  end

  let(:repo) do
    Class.new(ROM::Repository[:posts]) do
      relations :users

      def all
        posts.combine_parents(one: users.as_entity).as(Post).to_a
      end
    end.new(rom)
  end

  before { create_post(title: 'post_title', user_name: 'Sergiy') }

  it 'fetches 1 record' do
    expect(repo.all.count).to eq 1
  end

  it 'builds User' do
    expect(repo.users.as(:entity).first).to be_a(User)
  end

  let(:first) { repo.all.first }

  it 'builds Post struct' do
    expect(first).to be_a(Post)
    expect(first.title).to eq 'post_title'
    expect(first.user.name).to eq 'Sergiy'
  end

  def create_post(attrs)
    user_id = rom.relations[:users].insert(nname: attrs.fetch(:user_name))
    rom.relations[:posts].insert(title: attrs.fetch(:title), user_id: user_id)
  end
end

Let's assume as fact, that we need to map record from users table in order to create User entity. For simplicity, we just need to rename :nname into :name. And we want to combine posts relation with its parent users.

Test fails, because mapping isn't called, while repo.users.as(:entity) maps users to User struct without problems

3rd revision works fine