Question

For instance,

s1 = Student.new(1, "Bob", "Podunk High")
hash[1] = s1
puts hash[1].name    #produces "Bob"
s1.id = 15
puts hash[15].name   #produces "Bob"
puts hash[1].name    #fails

This is not exactly Hash-like behavior and insertions with the wrong key still needs to be defined.

While I can certainly roll my own container that behaves this way but it will be hard to make it fast, ie not search through the whole container every time [] is called. Just wondering if someone smarter has already made something I can steal.

EDIT: Some good ideas below helped me focus my requirements:

1. avoid the O(n) lookup time

2. allow multiple containers to the same object (association not composition)

3. have different data types (eg. that might use name instead of id) without too much reimplementation

Answer 1

You can implement it yourself.

Look at the draft solution:

class Campus
  attr_reader :students
  def initialize
    @students = []
  end

  def [](ind)
    students.detect{|s| s.id == ind}
  end

  def <<(st)
    raise "Yarrr, not a student"   if st.class != Student
    raise "We already have got one with id #{st.id}" if self[st.id]
    students << st
  end
end

class Student
  attr_accessor :id, :name, :prop
  def initialize(id, name, prop)
    @id, @name, @prop = id, name, prop
  end
end

campus = Campus.new
st1 = Student.new(1, "Pedro", "Math")
st2 = Student.new(2, "Maria", "Opera")
campus << st1
campus << st2
campus[1]
#=> Student...id:1,name:pedro...
campus[2].name
#=> Maria
campus[2].id = 10
campus[2]
#=> error
campus[10].name
#=> Maria

Or you can play around Array class (or Hash, if you really need it):

class StrangeArray < Array
  def [](ind)
    self.detect{|v| v.id == ind} || raise "nothing found" # if you really need to raise an error
  end

  def <<(st)
    raise "Looks like a duplicate" if self[st.id]
    self.push(st)
  end
end

campus = StrangeArray.new
campus << Student.new(15, 'Michael', 'Music')
campus << Student.new(40, 'Lisa', 'Medicine')
campus[1]
#=> error 'not found'
campus[15].prop
#=> Music
campus[15].id = 20
campus[20].prop
#=> Music

etc

And after @tadman’s correct comment you can use reference to your hash right into your Student class:

class Student
  attr_accessor :name, :prop
  attr_reader :id, :campus
  def initialize(id, name, prop, camp=nil)
    @id, @name, @prop = id, name, prop
    self.campus = camp if camp
  end

  def id=(new_id)
    if campus
      rase "this id is already taken in campus" if campus[new_id]
      campus.delete id
      campus[new_id] = self
    end
    @id = new_id
  end

  def campus=(camp)
    rase "this id is already taken in campus" if camp[id]
    @campus = camp
    camp[@id] = self
  end
end

campus = {}
st1 = Student.new(1, "John", "Math")
st2 = Student.new(2, "Lisa", "Math", campus)
# so now in campus is only Lisa
st1.campus = campus
# we've just pushed John in campus
campus[1].name
#=> John
campus[1].id = 10
campus[10].name
#=> John