Initialize from within instance method call

Question

In my Rails application. I have a module in which I am overriding .eql? method like below

# lib/item_util.rb

module ItemUtil
  def eql?(item, field: "cost", op: "==")
    item.send(field).present? &&
      item.send(field).send(op, self.send(field))
  end
end

Which is included in Item model

# app/models/item.rb

class Item < ApplicationRecord
  include ItemUtil
end

In my controller I want to check various conditions based on the attribute values. Ex:

@item1 = Item.find(:id)
@item2 = Item.find(:id)

@item1.eql?(@item2, field: "discount", op: ">") # @item2.discount > @item1.discount
@item2.eql?(@item1, op: "<=") # @item1.cost <= @item2.cost

# ...

All this is working fine and I want to write ItemUtil module in a neater way like below:

  module ItemUtil
    attr_accessor :item, :field

    def initialize(item, field: "cost")
      @item = item
      @field = field
    end

    def eql?(item, field: "cost", op: "==")
      new_item.present? && new_item.send(op, current_item)
    end

    def new_item
      @item.send(@field)
    end

    def current_item
      self.send(@field)
    end
  end

This returns TypeError (nil is not a symbol nor a string) for @field inside new_item method, as initialize wasn't invoked anywhere

Traceback (most recent call last):
        2: from lib/item_util.rb:12:in `eql?'
        1: from lib/item_util.rb:17:in `new_item'
TypeError (nil is not a symbol nor a string)

but I dont want to change the way I call .eql? on object i.e., I'd like to keep these lines intact

@item1.eql?(@item2, field: "discount", op: ">") # @item2.discount > @item1.discount
@item2.eql?(@item1, op: "<=") # @item1.cost <= @item2.cost

How do I get the new_item and current_item return the desired output?
How to invoke initialize method within .eql? method?
Is there an alternate approach for this?
Is there a way to access parameters in modules similar to before_action(in controllers)?

Note that equality is expected to be symmetric, i.e. a.eql?(b) usually implies b.eql?(a). This is no longer true the way you implemented eql?. — Stefan
– Stefan, Commented Oct 12, 2021 at 12:32

Tom Lord · Accepted Answer · 2021-10-12 11:26:07Z

2

You cannot instantiate an instance of a module. (You can instantiate an instance of a class, but not a module.)

In fact, what you are actually doing here is overriding the definition of Item#initialize!!

Sticking with your general design pattern, what I would suggest is to abstract this logic into a new class - like, say, ItemComparator:

class ItemComparator
  attr_reader :item, :other

  def initialize(item, other)
    @item = item
    @other = other
  end

  def eql?(field:, op:)
    item_field = item.send(field)
    other_field = other.send(:field)

    other_field.present? && item_field.send(op, other_field)
  end
end

module ItemUtil
  def eql?(other, field: "cost", op: "==")
    ItemComparator.new(self, other).eql?(field: field, op: op)
  end
end

class Item < ApplicationRecord
  include ItemUtil
end

answered Oct 12, 2021 at 11:26

Tom Lord

28.4k5 gold badges54 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Tom Lord Over a year ago

However, there are some aspects of this design that are dubious.... For instance, why is this a desired design?? @item1.eql?(@item2, field: "discount", op: ">") -- You want to check if two items are equal, judging by whether one field is actually greater than the other?! That's super misleading.

Tom Lord Over a year ago

At a bare minimum, perhaps this method should be called something like compare? instead of eql?!!

Manjunath P Over a year ago

I wanted to perform <=> operation within the method. After your suggestion compare? makes more sense. Thanks for the solution, its very helpful.

Manjunath P Over a year ago

Wouldn't it be possible to instantiate even if I wrapped my module code inside class << self ... end block within module?

Tom Lord Over a year ago

@ManjunathP You can not instantiate an instance of a module. This is the fundamental difference between a class and a module.

Collectives™ on Stack Overflow

Initialize from within instance method call

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related