How to use polymorphism to remove a switch statement which compares strings?

Question

I am new to Ruby, so let me describe the context of my problem first: I have a json as input which has the following key / value pair:

{
 "service": "update"
}

The value has many different values for example: insert,delete etc. Next there is a method x which handles the different requests:

def x(input)

  case input[:service]      
  services = GenericService.new

  when "update"
    result = services.service(UpdateService.new,input)
  when "insert"
    result = services.service(InsertService.new,input)
  when "delete"
    result = services.service(DeleteService.new,input)
  ....
  ....
  else
    raise "Unknown service"
  end
  puts JSON.pretty_generate(result)    
end

What is bothering me is that I still need to use a switch statement to check the String values (reminds me of 'instance of' ugh..). Is there a cleaner way (not need to use a switch)?

Finally I tried to search for an answer to my question and did not succeed, if however I missed it feel free to comment the related question.

Update: I was thinking to maybe cast the string to the related class name as follows: How do I create a class instance from a string name in ruby? and then call result = services.services(x.constantize.new,input) , then the class names ofcourse needs to match the input of the json.

score 3 · Accepted Answer · answered Apr 28 '16 at 11:35

3

You can try something like:

def x(input)

  service_class_name = "#{input[:service].capitalize}Service"
  service_class = Kernel.const_get(service_class_name)
  service_class.new(input).process

end

In addition you might want to check if this is a valid Service class name at all.

I don't understand why you want to pass the service to GenericService this seems strange. let the service do it's job.

answered Apr 28 '16 at 11:35

Pascal

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GenericService becomes neglect now, because I can directly call update,insert etc. – M. Suurland Apr 28 '16 at 11:50
how can it be achieved to limit the Kernal.const_get to a specific module Y? – M. Suurland Apr 28 '16 at 14:01
Nevermind got it :) Object.const_get(module_name).const_get(class_name).new – M. Suurland Apr 28 '16 at 14:24

score 1 · Answer 2 · answered Apr 28 '16 at 11:32

If you're trying to instatiate a class by it's name you're actually speaking about Reflection rather than Polymorphism.

In Ruby you can achieve this in this way:

byName = Object.const_get('YourClassName')

or if you are in a Rails app

byName= 'YourClassName'.constantize

Hope this helps

ABrowne · Answer 3 · 2016-04-28T11:38:11.247

0

Just first thoughts, but you can do:

eval(services.service("#{input[:service].capitalize}Service.new, #{input})") if valid_service? input[:service]

def valid_service?
    w%(delete update insert).include? input[:service]
end

As folks will no doubt shout, eval needs to be used with alot of care

edited Apr 28 '16 at 11:38

answered Apr 28 '16 at 11:36

ABrowne

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Neverever use "eval". It is "evil". – Pascal Apr 28 '16 at 11:36
@pascalbetz - I would agree you can get yourself into alot of trouble with eval, I prefer the the const_get as it'll just throw an error if you try grabbing something that doesn't exist, hence the need for the sanity check on inbound. – ABrowne Apr 28 '16 at 11:43

How to use polymorphism to remove a switch statement which compares strings?

3 Answers3