How to call methods dynamically using string of method name

by Axel Kee 15 February 2021

I was working on a feature for my Rails app, which allows merchant to set fulfillment times on each day like this :

fulfillment_time

The main feature is that once an order is placed, the customer can’t cancel the placed order after the fulfillment time starts on that day (or on tomorrow).

The fulfillment start times are saved in different columns like this :

create_table "fulfillments", force: :cascade do |t|
  t.time "monday_start_time", default: "2000-01-01 00:00:00", null: false
  t.time "tuesday_start_time", default: "2000-01-01 00:00:00", null: false
  t.time "wednesday_start_time", default: "2000-01-01 00:00:00", null: false
  t.time "thursday_start_time", default: "2000-01-01 00:00:00", null: false
  t.time "friday_start_time", default: "2000-01-01 00:00:00", null: false
  t.time "saturday_start_time", default: "2000-01-01 00:00:00", null: false
  t.time "sunday_start_time", default: "2000-01-01 00:00:00", null: false
end

To check if the fulfillment time has started, my app will get the current day of week using DateTime.current.strftime('%A').downcase . If today is wednesday, then it will return wednesday .

The question now is that how do I access value of different column based on different day of week? Say I want to get the value of thursday_start_time if today is thursday ; or the value of sunday_start_time is today is sunday .

Fortunately, Ruby’s metaprogramming feature allows us to call methods dynamically by just passing the method name into public_send(method_name) or send(method_name) .

Say we have a class like this :

class Duck
  def make_noise
    puts "quack"
  end
  
  private

  def jump
    puts "can't jump"
  end
end

We can call the make_noise method by calling Duck.new.public_send("make_noise"), this is equivalent to calling Duck.new.make_noise .

For the jump method, we would need to use send instead, as it is a private method, Duck.new.send("jump")

For the fulfillment start time, we can then pass in the day dynamically like this :

# this will return 'monday' if today is monday, or 'tuesday' if today is tuesday etc..
day_of_week = DateTime.current.strftime('%A').downcase

# if today is monday, it will call 'monday_start_time', etc..
start_time = fulfillment.public_send("#{day_of_week}_start_time")

Sweet!

Alternatively, you can also pass in symbol instead of string : public_send(:make_noise) .

Passing parameters to public_send / send

If you would like to send parameters to the method, you can pass them after the method name :

class Animal
  def make_noise(noise, times)
    1..times.each do 
      puts noise
    end
  end
end

Animal.new.public_send('make_noise', 'quack', 3)
# quack
# quack
# quack

Reference

Ruby Doc’s public_send documentation

Ruby Doc’s send documentation

How to call methods dynamically using string of method name

Passing parameters to public_send / send

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