Ruby how to return instance evaled array

Question

I have method in my rails model which returns anonymous class:

def today_earnings
Class.new do
  def initialize(user)
    @user = user
  end

  def all
    @user.store_credits.where(created_at: Time.current.beginning_of_day..Time.current.end_of_day)
  end

  def unused
    all.map { |el| el.amount - el.amount_used }.instance_eval do
      def to_f
        reduce(:+)
      end
    end
  end

  def used
    all.map(&:amount_used).instance_eval do
      def to_f
        reduce(:+)
      end
    end
  end
end.new(self)

end

I want to achieve possibility to chain result in that way user.today_earning.unused.to_f and I have some problems with instance eval because when I call to_f on result it's undefined method, I guess it is due to ruby copying returned value so the instance gets changed, is it true? And if I'm correct how can I change the code to make it work. Also I'm wondering if making new model can be better solution than anomyous class thus I need advice if anonymous class is elegant in that case and if so how can I add to_f method to returned values

I don't think you need this complexity. A new model would probably be cleaner. Also, you don't calculate or return anything in instance_eval, you only define to_f, so you then try to call the method to_f on the :to_f Symbol. You might try to add self at the end of the instance_eval block but it feels like a hack. — Eric Duminil
– Eric Duminil, Commented Feb 13, 2020 at 12:09
I assume that you have a store_credits table with amount and amount_used columns? If so, the obvious solution would be a database query for getting the results. — Stefan
– Stefan, Commented Feb 13, 2020 at 15:43

lokanadham100 · Accepted Answer · 2020-02-13 14:31:56Z

1

Yes, Anonymous class makes the code much complex. I would suggest a seperate class. It will solve 2 problems here.

defining some anonymous class again and again when we call the today_earnings method.
Readability of the code.

Now coming to actual question, you can try something similar to hash_with_indifferent_access. The code looks as follows.

class NumericArray < Array
   def to_f
     reduce(:+)
   end
end

Array.class_eval do
   def with_numeric_operations
      NumericArray.new(self)
   end
end

Usage will be:

Class Earnings
  def initialize(user)
    @user = user
  end

  def all
    @user.store_credits.where(created_at: Time.current.beginning_of_day..Time.current.end_of_day)
  end

  def unused
    all.map { |el| el.amount - el.amount_used }.with_numeric_operations
  end

  def used
    all.map(&:amount_used).with_numeric_operations
  end
end

answered Feb 13, 2020 at 14:31

lokanadham100

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1 Comment

max Over a year ago

Subclassing Array or any of the other classes in the core (String, Numeric etc.) is a bad idea. They are written in C and do not behave like normal ruby classes. Better would be to use SimpleDelegator and wrap the array. words.steveklabnik.com/beware-subclassing-ruby-core-classes

max · Accepted Answer · 2020-02-13 16:57:21Z

1

This looks like a "clever" but ridiculously over-complicated way to do something that can be simply and efficiently done in the database.

User.joins(:store_credits)
    .select(
       'users.*',
       'SUM(store_credits.amount_used) AS amount_used',
       'SUM(store_credits.amount) - amount_used AS unused',
    )
    .where(store_credits: { created_at: Time.current.beginning_of_day..Time.current.end_of_day })
    .group(:id)

answered Feb 13, 2020 at 16:57

max

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