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

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 bordering 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.