gistfile1.rb

require 'rubygems'
require 'bundler/setup'
require 'rom'
require 'rom-sql'

ROM.setup(:sql, 'postgres://localhost/datsu_development')

class Players < ROM::Relation[:sql]
  register_as :players
  dataset :players
end

class Throws < ROM::Relation[:sql]
  register_as :throws
  dataset :throws

  def for_users(users)
    where(player_id: users.map { |user| user[:id] }).order(:order).limit(2)
  end
end

class PlayersMapper < ROM::Mapper
  relation :players
  register_as :with_throws

  reject_keys true
  attribute :id
  attribute :name

  combine :throws, on: { id: :player_id }
end

ROM.finalize

rom = ROM.env

puts rom.relation(:players).combine(rom.relation(:throws).for_users).as(:with_throws).to_a

# Returns an empty hash as throw
{:id=>1, :name=>"David", :throws=>[{}]}
{:id=>2, :name=>"Thomas", :throws=>[{}]}

# Removing `reject_keys true` produces the correct output
{:id=>1, :created_at=>2015-05-09 12:33:50 +0200, :updated_at=>2015-05-09 12:33:50 +0200, :name=>"David", :throws=>[{:id=>57, :match_id=>5, :player_id=>1, :is_bust=>false, :darts=>"[\"5\",\"20\",\"T20\"]", :order=>0, :created_at=>2015-05-09 12:33:50 +0200, :updated_at=>2015-05-09 12:46:10 +0200, :score=>85, :score_before_throw=>301, :score_after_throw=>216}]}
{:id=>2, :created_at=>2015-05-09 12:33:50 +0200, :updated_at=>2015-05-09 12:33:50 +0200, :name=>"Thomas", :throws=>[{:id=>58, :match_id=>5, :player_id=>2, :is_bust=>false, :darts=>"[\"2\",\"5\",\"1\"]", :order=>1, :created_at=>2015-05-09 12:33:50 +0200, :updated_at=>2015-05-09 12:46:10 +0200, :score=>8, :score_before_throw=>301, :score_after_throw=>293}]}