How do I get a list of all subclasses without instantiating first in Ruby

Question

If I have 4 classes with the following hierarchy:

class MainClass < ActiveRecord::Base
  ...
end

class SubClassA < MainClass
  ...
end

class SubClassB < MainClass
  ...
end

class SubClassC < MainClass
  ...
end

How can I get a list of the subclasses of MainClass without going through and creating instances of each of the other classes?

In a fresh IRB session I can go in and say

irb(main)> MainClass.descendants
=> []

However if I go through and create instances of each subclass I'll see the following

irb(main)> SubClassA.new
=> #<SubClassA ...>
irb(main)> SubClassB.new
=> #<SubClassB ...>
irb(main)> SubClassC.new
=> #<SubClassC ...>
irb(main)> MainClass.descendants
=> [SubClassA(...), SubClassB(...), SubClassC(...)]

I'm basically looking for a way to programmaticly supply all subclasses so in the future when I want to add SubClassD, SubClassE, etc., I won't have to worry that each one is instantiated in the code before the user can see them.

I tried on rail console (rails 4.1.2). I am getting this (and not an empty list like you) :- irb(main):012:0> MainClass.descendants => [SubClassA(Table doesn't exist), SubClassB(Table doesn't exist)] — rohan, Jan 15 '16 at 18:51
Interesting. Originally, I split my subclasses out into their own files (sub_class_a.rb, sub_class_b.rb, sub_class_c.rb) all at the same level and I was getting what I detailed above. When I consolidated the classes all in the same main_class.rb, the descendants method worked as you said. — user1535152, Jan 15 '16 at 19:02

score 11 · Accepted Answer · answered Jan 15 '16 at 19:09

11

This is an artefact of development mode only loading classes when first referenced: those files haven't been read by the interpreter yet - as far as ruby is concerned the classes genuinely do not exist yet

A workaround is to put

require_dependency "subclass_a"
require_dependency "subclass_b"
....

At the bottom the file for main class (outside the class definition)

answered Jan 15 '16 at 19:09

Frederick Cheung

83,189
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2

Is this solution RAILS specific? – Wand Maker Jan 15 '16 at 19:11
1

It is - both the descendants method that is misbehaving and require_dependency are rails specific. – Frederick Cheung Jan 15 '16 at 19:13
Thank you! This did it. It wasn't even in my mind that the files weren't being loaded in development – user1535152 Jan 15 '16 at 19:45

score 8 · Answer 2 · edited May 23 '17 at 12:16

8

There are several ways you can achieve this. Either by using a gem such as the descendants_tracker:

class MainClass < ActiveRecord::Base
  extend DescendantsTracker
end

class SubClassA < MainClass 
end

...

MainClass.descendants # => [SubClassA]

Or you could create a custom one in your parent class like this:

class  MainClass < ActiveRecord::Base
  def self.descendants
    ObjectSpace.each_object(Class).select { |klass| klass < self }
  end
end

class SubClassA < MainClass 
end

class SubClassB  < MainClass 
end

puts MainClass.descendants 

#=> SubClassA
    SubClassB

You can also check for levels of dependencies. If you want an array of the direct child of MainClass for example you could use the .subclasses class method

MainClass.subclasses  # => [SubClassA, SubClassB,...]

Now if you want more than one level depth of child classes use .descendants. You can find an example here

edited May 23 '17 at 12:16

Community

1
1

answered Jan 15 '16 at 18:50

Cyzanfar

6,997
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`< ActiveRecord::Base` may not be relevant for this answer. – Wand Maker Jan 15 '16 at 19:10
Meaning the `.subclasses` and `.descendants` methods can't be called on an ActiveRecord model? – Cyzanfar Jan 15 '16 at 19:12
Right, I was trying to adapt my answer to the user's question that is all. – Cyzanfar Jan 15 '16 at 19:15
1

Sorry, did not realize that. Its okay then. – Wand Maker Jan 15 '16 at 19:16
No worries, it was a genuine comment :) – Cyzanfar Jan 15 '16 at 19:17

Cary Swoveland · Answer 3 · 2016-01-16T02:37:38.520

You could use ObjectSpace#each_object to compile a list of MainClass's children:

Suppose:

class MainClass            ; end
class SubClassA < MainClass; end
class SubClassB < MainClass; end
class SubClassC < SubClassB; end

The fact that MainClass may be a subclass is irrelevant.

ObjectSpace.each_object(Class).select { |o| o < MainClass }.
  tap { |siblings| siblings.reject! { |klass| siblings.any? { |k| klass < k } } }
  #=> [SubClassB, SubClassA]

How do I get a list of all subclasses without instantiating first in Ruby

3 Answers3