Factory methods in Ruby

ruby factory
Peter picture Peter · Oct 4, 2009 · Viewed 20.8k times · Source

What is the slickest, most Ruby-like way to have a single constructor return an object of the appropriate type?

To be more specific, here's a dummy example: say I have two classes Bike and Car which subclass Vehicle. I want this:

Vehicle.new('mountain bike')  # returns Bike.new('mountain bike')
Vehicle.new('ferrari')        # returns Car.new('ferrari')

I've proposed a solution below, but it uses allocate which seems way too implementation-heavy. What are some other approaches, or is mine actually ok?

Answer

DigitalRoss picture DigitalRoss · Oct 4, 2009

If I make a factory method that is not called1 new or initialize, I guess that doesn't really answer the question "how do I make a ... constructor ...", but I think that's how I would do it...

class Vehicle
  def Vehicle.factory vt
    { :Bike => Bike, :Car => Car }[vt].new
  end
end

class Bike < Vehicle
end

class Car < Vehicle
end

c = Vehicle.factory :Car
c.class.factory :Bike

1. Calling the method factory works really well in this instructional example but IRL you may want to consider @AlexChaffee's advice in the comments.

Related questions

Populating an association with children in factory_girl

I have a model Foo that has_many 'Bar'. I have a factory_girl factory for each of these objects. The factory for Bar has an association to Foo; it will instantiate a Foo when it creates the Bar. I'd …

Read More
JSON data for rspec tests

I'm creating an API that accepts JSON data and I want to provide testing data for it. Is there anything similar to factories for JSON data? I would like to have the same data available in an object and in …

Read More
How to check if a value exists in an array in Ruby

I have a value 'Dog' and an array ['Cat', 'Dog', 'Bird']. How do I check if it exists in the array without looping through it? Is there a simple way of checking if the value exists, nothing more?

Read More