Enumerable changes my `to_json` behavior

Question

I have a rails application and a class I've wrote as a part of it (not an ActiveRecord or anything..). The data is stored in simple instance variables (string, integers, arrays...)

When I invoke to_json on an instance of it I get what I expect to. A JSON object, containing all instance variables as JSON objects too.

However, when I add include Enumerable to the class definition, the behavior of to_json changes and I get an empty object: "[]"

Any idea why is that? Does Enumerable somehow defined or undefines something that affects to_json?

Thanks!

Answer 1

So, what happens is:

Rails loads in ActiveSupport. ActiveSupport injects (monkey patches) these as_json methods into several classes and modules, including Enumerable:

module Enumerable
  def as_json(options = nil) #:nodoc:
    to_a.as_json(options)
  end
end

You're probably returning nothing for the each method Enumerable requires you to have, so to_a returns [], and an empty array gets converted into the String "[]".

What you can do here, is, to bind your object into a non-enumerable inherited class, and use its .as_json method.

Like this:

class A
  def as_json(*)
    Object.instance_method(:as_json).bind(self).call
  end  
end

Demo:

➜ pry
require 'active_support/all'
=> true

class A
  def initialize
    @a = 1
  end  
end  
=> nil

A.new.to_json
=> "{\"a\":1}"

class A
  include Enumerable
  def each
  end
end
=> nil

A.new.to_json
=> "[]"

class A
  def as_json(*)
    Object.instance_method(:as_json).bind(self).call
  end  
end  
=> nil

A.new.to_json
=> "{\"a\":1}"