For the article at medium.com

happymapper2.rb

class Student
  include HappyMapper
  tag 'student'
  has_one :name, String
  has_one :school_id, String, tag: 'schoolId'
end

class School
  include HappyMapper
  tag 'school'
  has_one :id, String
  has_one :name, String
end

let(:schools) { School.parse(xml) }
let(:students) { Student.parse(xml) }

it do
  school_id = schools.find { |school| school.name == 'Second school' }.id
  student_names = students.select { |student| student.school_id == school_id }.map(&:name)
  expect(student_names).to match(%w(Alex Olivia))
end