How to modify OpenStruct in the instantiated class

Question

I have the following code where I instantiate an object that returns an OpenStruct result.

require 'ostruct'

class TestModel
  attr_accessor :result

  def initializer
  end

  def result
    OpenStruct.new(successful?: false)
  end

  def successful!
    result.send('successful?', true)
  end
end

I want the class to work so that I could modify any attributes of my result on the fly.

test = TestModel.new

test.result
=> #<OpenStruct successful?=false>

test.result.successful!
=> #<OpenStruct successful?=true>

This syntax comes from the official OpenStruct page, and it works just on its own but not inside an instantiated class - https://ruby-doc.org/stdlib-2.5.3/libdoc/ostruct/rdoc/OpenStruct.html

result.send('successful?', true)

I've also tried to use lambda but to no avail

  def result
    OpenStruct.new(successful?: false, :successful! => lamdba {self.uccessful? = true})
  end

Any ideas? I really want to know how to do that.

score 1 · Accepted Answer · answered Feb 23 '21 at 06:20

OpenStruct requires you to use Object#send or Hash-like keys to make use of predicate symbols. The documentation says:

Hash keys with spaces or characters that could normally not be used for method calls (e.g. ()[]*) will not be immediately available on the OpenStruct object as a method for retrieval or assignment, but can still be reached through the Object#send method or using [].

In addition, it's unclear why you want to define @result as writable, or why you would override the getter method so that TestModel#result always returns false. That's probably causing at least part of your problem.

Instead, I'd rewrite the code as follows:

require 'ostruct'

class TestModel 
  attr_reader :result

  def initialize
    @result = OpenStruct.new :successful? => nil
  end

  def unsuccessful
    @result.send "successful?=", false
  end
    
  def successful!
    @result.send "successful?=", true
  end
end

test_model = TestModel.new
#=> #<TestModel:0x00007faf5c1b9528 @result=#<OpenStruct successful?=nil>>

test_model.result
#=> nil

test_model.successful!
#=> true

test_model.result
#=> #<OpenStruct successful?=true>

test_model.unsuccessful
#=> false

test_model.result
#=> #<OpenStruct successful?=false>

You could certainly initialize the struct member as false rather than nil if you prefer, but I think this is semantically clearer. Your mileage in that regard may vary.

How to modify OpenStruct in the instantiated class

1 Answers1