Question

Let’s say I have a class like:

class Basket < ActiveRecord::Base
  has_many :fruits

Where “fruits” is an STI base class having subclasses like “apples”, “oranges”, etc…

I’d like to be able to have a setter method in Basket like:

def fruits=(params)
  unless params.nil?
    params.each_pair do |fruit_type, fruit_data|
      fruit_type.build(fruit_data)
    end
  end
end

But, obviously, I get an exception like:

NoMethodError (undefined method `build' for "apples":String)

A workaround I thought of works like this:

def fruits=(params)
  unless params.nil?
    params.each_pair do |fruit_type, fruit_data|
      "#{fruit_type}".create(fruit_data.merge({:basket_id => self.id}))
    end
  end
end

But that causes the Fruit STI object to be instantiated before the Basket class, and so the basket_id key is never saved in the Fruit subclass (because basket_id doesn’t exist yet).

I’m totally stumped. Anyone have any ideas?

Answer 1

Instead of adding a setter method in Basket, add it in Fruit:

class Fruit < ActiveRecord::Base
  def type_setter=(type_name)
    self[:type]=type_name
  end
end

Now you can pass the type in when you build the object through an association:

b = Basket.new
b.fruits.build(:type_setter=>"Apple")

Note that you can’t assign :type this way, since it is protected from mass assignment.

EDIT

Oh, you wanted to run different callbacks depending on the subclass? Right.

You could do this:

fruit_type = "apples"
b = Basket.new
new_fruit = b.fruits << fruit_type.titleize.singularize.constantize.new
new_fruit.class # Apple

or define a has_many association for each type:

require_dependency 'fruit' # assuming Apple is defined in app/models/fruit.rb

class Basket
  has_many :apples
end

then

fruit_type = "apples"
b = Basket.new
new_fruit = b.send(fruit_type).build
new_fruit.class # Apple