Activeadmin formtastic dynamic select - ruby-on-rails

Question

I would like to make a dynamic select option via Activeadmin's formtastic like so:
form do |f|
f.inputs "Exam Registration Details" do
f.input :user_id, :as => :select, :collection => User.where(:admin => 'false')
#selects user from list. WORKING
f.input :student_id, :as => :select, :collection => Student.joins(lessons: :user)
#collection of students will change to students who have lessons with chosen user. NOT WORKING, returns all students who have lessons.
f.input :lesson_id, :as => :select, :collection => Lesson.joins(:student, :user)
#collection of lessons will change to reflect lessons connected by chosen user and student. NOT WORKING, returns all lessons.
end
f.buttons
end
My amateurish code is obviously not working as I intended it to. What changes should I make?
I have 4 models as below:
class Student < ActiveRecord::Base
has_many :lessons
has_many :users, through: :lessons
has_many :exam_registrations, through: :lessons
class Lesson < ActiveRecord::Base
belongs_to :user
belongs_to :student
belongs_to :exam_registration
class User < ActiveRecord::Base
has_many :lessons
has_many :students, through: :lessons
has_many :exam_registrations, through: :lessons
class ExamRegistration < ActiveRecord::Base
has_many :lessons
has_many :users, through: :lessons
has_many :students, through: :lessons

Answer 1

SOLVED
For anyone else wrestling with the same problem, look at this railscast
here's how I implemented multiple dynamic select menus in activeadmin:
config/initializers/active_admin.rb
config.register_javascript 'exam_registrations.js.coffee'
app/admin/exam_registrations.rb
form do |f|
f.inputs "Exam Registration Details" do
f.input :user_id, :label => 'Teacher', :as => :select, :collection => User.where(:admin => 'false', :active => true).order(:name), :include_blank => true
f.input :student_id, :hint => 'Students grouped by teacher names', :as => :select, :collection => option_groups_from_collection_for_select(User.where(:admin => false, :active => true).order(:name), :students, :name, :id, :name)
f.input :lesson_id, :hint => 'Lessons grouped by student names', :as => :select, :collection => option_groups_from_collection_for_select(Student.where(:active => true).order(:name), :lessons, :name, :id, :name)
end
f.buttons
end
app/assets/javascripts/exam_registrations.js.coffee
#first menu
jQuery ->
$('#exam_registration_student_id').parent().hide()
students = $('#exam_registration_student_id').html()
$('#exam_registration_user_id').change ->
user = $('#exam_registration_user_id :selected').text()
escaped_user = user.replace(/([ #;&,.+*~\':"!^$[\]()=>|\/#])/g, '\\$1')
options = $(students).filter("optgroup[label='#{escaped_user}']").html()
if options
$('#exam_registration_student_id').html(options)
$('#exam_registration_student_id').parent().show()
else
$('#exam_registration_student_id').empty()
$('#exam_registration_lesson_id').empty()
# second menu
$('#exam_registration_lesson_id').parent().hide()
lessons = $('#exam_registration_lesson_id').html()
$('#exam_registration_student_id').click ->
student = $('#exam_registration_student_id :selected').text()
escaped_student = student.replace(/([ #;&,.+*~\':"!^$[\]()=>|\/#])/g, '\\$1')
options = $(lessons).filter("optgroup[label='#{escaped_student}']").html()
if options
$('#exam_registration_lesson_id').html(options)
$('#exam_registration_lesson_id').parent().show()
else
$('#exam_registration_lesson_id').empty()
restart the server and the menus work!