rom = ROM.container(:sql, 'sqlite::memory') do |conf|
  conf.gateways[:default].use_logger(Logger.new($stdout))

  conf.default.create_table(:users) do
    primary_key :id
    column :login, String
  end

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

  conf.relation(:users) do
    schema(:users, infer: true) do
    end
  end

  conf.relation(:tasks) do
    schema(:tasks, infer: true) do
    end

    def for_users(users)
      # where(employee_id: users.map { |tuple| tuple[:employee_id] })
      where(user_id: 1)
    end
  end
end

user = rom.relations[:users]
  .changeset(:create, login: 'jane@doe.org')
  .commit

rom.relations[:tasks].changeset(
  :create,
  [
    { name: 'ble ble', user_id: user[:id] },
    { name: 'ble', user_id: user[:id] }
  ]
).commit

p rom.relations[:users].combine_with(rom.relations[:tasks].for_users).to_a
# It fetches both relations:
#  INFO -- : (0.000088s) SELECT `users`.`id`, `users`.`login` FROM `users` ORDER BY `users`.`id`
#  INFO -- : (0.000106s) SELECT `tasks`.`id`, `tasks`.`user_id`, `tasks`.`name` FROM `tasks` WHERE (`user_id` = 1) ORDER BY `tasks`.`id`

# buth finally fails probably on mapping:
/work/.rvm/gems/ruby-2.3.4@isa/gems/rom-mapper-1.1.0/lib/rom/processor/transproc.rb:40:in `inject_union_value': [[:id], [:user_id], [:name]] attribute: block is required for :from with union value. (ROM::MapperMisconfiguredError)
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/function.rb:47:in `call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/function.rb:47:in `call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/composite.rb:30:in `call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/array.rb:42:in `block in map_array'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/array.rb:42:in `map'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/array.rb:42:in `map_array'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/function.rb:47:in `call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/transproc-1.0.2/lib/transproc/function.rb:47:in `call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/rom-mapper-1.1.0/lib/rom/mapper.rb:95:in `block in call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/rom-mapper-1.1.0/lib/rom/mapper.rb:95:in `each'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/rom-mapper-1.1.0/lib/rom/mapper.rb:95:in `reduce'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/rom-mapper-1.1.0/lib/rom/mapper.rb:95:in `call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/rom-core-4.1.0/lib/rom/relation/combined.rb:64:in `call'
        from /work/.rvm/gems/ruby-2.3.4@isa/gems/rom-core-4.1.0/lib/rom/relation/materializable.rb:13:in `to_a'