Dynamically add methods in ruby with method_missing

*Note if you need to define methods dynamicallly I found a better way to do so with the define_method, but if you’re interested in learning about overriding method_missing or another way of dynamically adding methods in ruby, please read on.

When you call a method that doesn’t exist in ruby, internally ruby will call it’s own method_missing method, which will raise the undefined method error:

hello_world
=> `<main>': undefined local variable or method `hello_world' for main:Object (NameError)

But if needed you can override the default method_missing implementation with behavior to suite your applications needs.

For example you could use method_missing to catch unknown methods and define them dynamically. In the implementation below ActiveRecordImplementation initializes an empty hash of attributes which method_missing will reference and add to if the attribute does not currently exist on the object.

class ActiveRecordImplementation
  attr_reader :attributes

  def initialize
    @attributes = {}
  end

  def method_missing(name, *args, &block)
    if name.end_with?("=")
      @attributes[name.to_s[0..-2].to_sym] = args.first
    else
      @attributes[name]
    end
  end
end

ActiveRecordImplementation.name = "Jimmy Bones"

If it’s not clear, what makes this dynamic is our code is defining the attribute methods for us inside of method_missing as it runs rather than us declaring them within our code.

A note of caution in using this, if you’re not careful overriding method_missing can cause other issues with ruby behavior, a noted example is with respond_to?. It may be helpful to include a default conditional that just calls the ruby implementation by calling super in this case.