Metaprogramming: #const_set

Flatiron School / 14 December 2012

The following is a guest post by Kevin Curtin and originally appeared on his blog. Kevin is currently a student a The Flatiron School. You can learn more about him here, or follow him on twitter here.

This code sample is from the Proto gem

The Goal

To dynamically create classes based on the parameters of a method

The Code

The Breakdown

We are used to creating classes like this:

Since everything is an object in ruby, it turns out that we can create a class as we would any other object: new_class = This instantiates a new class that inherits from whatever you pass in as the parameter. In this case, OpenStruct.

One of the requirements for creating the return objects for Proto is that they are namespaced. We don’t want the constants we return to conflict with names that already exist in the code base. Modules have a #const_set method that allows you define a new constant within the module. It accepts two arguments, the constant name and the value. Since we are creating a new class, we’ll pass in the name of our constant that was passed in with the initial method call and then pass in the class we just created:

If, for example, the user has passed in ‘Tweet’ to the .create_return_objectsmethod, they will get back Proto::Tweet objects.

We can now access this new class using the #const_get method, newing up the class and passing in our attributes. Since the class we created inherits from OpenStruct and is just a value object, we don’t ever have to open it up and define behavior. Proto.const_get(name).new(hash)


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