Example of Ruby's Inject method.

inject_example.rb

Enrollment = Struct.new(:name, :type)

enrollments = [
  Enrollment.new('Don Draper', 'Teacher'),
  Enrollment.new('Peggy Olson', 'Student'),
  Enrollment.new('Pete Campbell', 'Student'),
  Enrollment.new('Roger Sterling', 'TA')
]

grouped_enrollments = enrollments.inject({}) do |result, enrollment|
  result[enrollment.type] ||= []
  result[enrollment.type] << enrollment
  
  result
end

#=> { 'Teacher' => [{name: 'Don Draper' ...}],
#     'Student' => [{name: 'Peggy Olson' ...}, {name: 'Pete Campbell' ...}],
#     'TA'      => [{name: 'Roger Sterling' ...}]}