Get class by name in Ruby

Question

I have two classes

class ClassOne def do_something [...] end end class ClassTwo def do_something [...] end end

I am getting a class name (either ClassOne or ClassTwo) from the database and I want to call do_something in that class

so I have

class_name = "ClassOne"

and I want to call ClassOne.do_something or ClassTwo.do_something if class_name is equals to "ClassTwo".

I can't do it using a simple if condition, I have many classes and am checking if the class exists before calling..

Is there a way to do it?

Sergio Tulentsev · Accepted Answer · 2012-03-08 15:42:32Z

6

For vanilla ruby:

Kernel.const_get('ClassOne').do_something

For Rails:

'ClassOne'.constantize.do_something

answered Mar 8, 2012 at 15:42

Sergio Tulentsev

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3 Comments

applechief Over a year ago

I am getting an undefined method 'do_something' for #<Class:0x9c9026c> (NoMethodError) Any idea why?

sgtFloyd Over a year ago

You need to def self.do_something to use it as a class method.

Jon M Over a year ago

Or leave the classes are they are and call Kernel.const_get('ClassOne').new.do_something / 'ClassOne'.constantize.new.do_something

tadman · Accepted Answer · 2012-03-08 15:46:02Z

2

Although you can convert any arbitrary string to a class using constantize from ActiveSupport if available, this could cause exceptions if users can submit the string in question. It might be safer to use a case:

case (with_class) when 'ClassOne', 'ClassTwo' with_class.constantize.do_something else raise "Um, what are you doing?" end

The same thing could be achieved with a Hash or Array defining valid classes and testing with either [] or include? accordingly.

answered Mar 8, 2012 at 15:46

tadman

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3 Comments

Andrew Marshall Over a year ago

Let alone potential security issues as the user could discover all your class names or potential reveal the value of constants if they can generate the string.

applechief Over a year ago

The strings are not user generated. In the future I might be adding new classes and I don't want to update this part of the code every time a new class is available..

tadman Over a year ago

Remember you can pass around classes as variables within the same Ruby process, so there's no need to keep them as strings. It's fairly common to do things like using_class = condition ? ClassA : ClassB and then operate on that variable as if it was the class itself. Duck-typing makes this possible.

slindsey3000 · Accepted Answer · 2012-03-08 16:26:56Z

-1

eval("#{classname}.do_something")

Note: you have to change your code to def self.do_something, otherwise these are instance methods. It looks like this is your intention.

answered Mar 8, 2012 at 16:26

slindsey3000

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1 Comment

tadman Over a year ago

No, please, no. eval is not something you just whip out to do something like this. This is best reserved for situations where there is simply no other way.

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