obj.method(argument) - ruby

Question

I am looking at this code:
class Student
attr_accessor :first_name, :last_name, :age
def initialize(first, last, age)
#first_name = first
#last_name = last
#age = age
end
def birthday
#age += 1
end
end
class ViewStudent
def initialize(student)
#student = student
end
def do_something
puts "Student name: #{#student.first_name} #{#student.last_name}"
end
end
class UpdateStudent
def initialize(student)
#student = student
end
def do_something
puts "What is the student's first name?"
#student.first_name = gets.chomp
puts "What is the student's last name?"
#student.last_name = gets.chomp
puts "Updated student: #{#student.first_name} #{#student.last_name}"
end
end
choices = [ViewStudent, UpdateStudent]
student = Student.new("John", "Doe", 18)
puts "Select 1 to view student or 2 to update student."
selection = gets.chomp.to_i
obj = choices[selection - 1]
obj = obj.new(student)
obj.do_something
In the last five lines, I understand that selection = gets.chomp.to_i converts the selection options to integers, but how does that work in tandem with obj = choices[selection - 1]?
I'm also not sure what obj = obj.new(student) and obj.do_something do. It looks like a local variable is being set to create a new object with student as the argument. However, obj isn't a class or method to call on?
I can also gather that obj.do_something calls the methods defined for both ViewStudent and UpdateStudent given the selection.
I saw this, but it doesn't answer my question.

Answer 1

obj = choices[selection - 1] just select ViewStudent if 1 and UpdateStudent if 2 from your array by index (choices[0] or choices[1]).
Then you creating an instance of selected class (ViewStudent.new or UpdateStudent.new) and call do_something method on this instance, because this methos difined in both classes:
obj = choices[selection - 1] # obj is ViewStudent or UpdateStudent class
obj = obj.new(student) # obj is ViewStudent or UpdateStudent instance
obj.do_something # call `do something` method on instance