NullObjectModels

Null Object Models is a gem that provides a finder method extension to ActiveRecord::Base objects.

1. find_or_null(id_or_array_of_ids, NullModelClass)

This returns either the found model(s) or null object(s) for that model if it's not found, given the ID(s). You can optionally pass the class that is used in place of any not found models.

If you don't pass a NullModelClass, the default NullModelObjects::Default class is used. You can implicitly set a class to be used as well, as we'll show below.

Defining a null object for your model can be done in one of two ways:

1. Define a class that matches the name Null${ModelName} and inherits from NullObjectModel::Default

eg. If your class is named 'Student', then define a class in the following way:

class NullStudent < NullObjectModels::Default
end

2. Define self.null_model and return the name of the class that is a null_model. This class must also inherit from NullModelObjects::Default. This way is useful if you do not want/can not have a class named Null${ModelName}

eg.

# student.rb
class Student
  def self.null_model
    MissingStudent
  end
end

# missing_student.rb
class MissingStudent < NullObjectModels::Default
end

The main value comes from defining defaults for your null models that allows you to handle logic in your application without throwing nil errors.

For example, say you have a Train model with a sound attribute in the database. With a null model implementation like:

class NullTrain < NullModelObjects::Default
  def sound
    "Choo choo!"
  end
end

you now prevent NilErrors on any code attempting to call #sound on null objects.

Eg. Trains with IDs (1,2) exist with the sound "Chugga Chugga". Train with ID 3 does not exist.

Train.find_or_null([1,2,3]).each |train|
  puts train.sound
end

# => "Chugga Chugga"
# => "Chugga Chugga"
# => "Choo choo!