Class: Module

Inherits:
Object show all
Defined in:
lib/adhearsion/foundation/relationship_properties.rb

Instance Method Summary collapse

Instance Method Details

#relationships(relationship_mapping) ⇒ Object

In OOP, relationships between classes should be treated as properties of those classes. Often, in a complex OO architecture, you’ll end up with many relationships that intermingle in monolithic ways, blunting the effectiveness of subclassing.

For example, say you have an Automobile class which, in its constructor, instantiates a new Battery class and performs some operations on it such as calling an install() method. Let’s also assume the Automobile class exposes a repair() method which uses a class-level method of Battery to diagnose your own instance of Battery. If the result of the diagnosis shows that the Battery is bad, the Automobile will instantiate a new Battery object and replace the old battery with the new one.

Now, what if you wish to create a new Automobile derived from existing technology: a HybridAutomobile subclass. For this particular HybridAutomobile class, let’s simply say the only difference between it and its parent is which kind of Battery it uses – it requires its own special subclass of Battery. With Automobile’s current implementation, its references to which Battery it instantiates and uses are embedded in the immutable method defintions. This HybridAutomobile needs to override which Battery its superclass’ methods use and nothing else.

For this reason, the Battery class which Automobile uses is semantically a property which others may want to override. In OOP theory, we define overridable properties in the form of methods and override those methods in the subclasses.

This method exposes one method which creates human-readable semantics to defining these relationships as properties. It’s used as follows:

class Automobile

relationship :battery => Battery
relationship :chassis => Chassis
# Other properties and instance methods here....

end

class HybridAutomobile < Automobile

relationship :battery => HybridBattery

end



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# File 'lib/adhearsion/foundation/relationship_properties.rb', line 36

def relationships(relationship_mapping)
  relationship_mapping.each_pair do |class_name, class_object|
    define_method(class_name) { class_object }
  end
end