Rails - Dynamically defining instance methods in a model

Question

I'm not sure if this can even be achieved, but here goes... :)

Lets assume two models, a Page model and a Field model. A Page has_many :fields and the Field model has two attributes: :name, :value

What I want to achieve is in the Page model to dynamically define instance methods for each Field#name returning the Field#value .

So if my page had a field with a name of "foobar", I would dynamically create a method like so:

def foobar
  fields.find_by_name("foobar").value
end

Can this be achieved? If so, please help?

Over to the rubyists...

Answer 1

You can override method_missing to achieve your goal:

def method_missing(method, *args, &block)
  super
rescue NoMethodError => e
  field = fields.find_by_name(method)
  raise e unless field
  field.value
end

Probably it's better to add prefix or suffix to your dynamic methods, eg:

def method_missing(method, *args, &block)
  if method.starts_with?('value_of_')
    name = method.sub('value_of_', '')
    field = fields.find_by_name(method)
    field && field.value
  else
    super
  end
end

And call it like:

page.value_of_foobar