message-chain.md

Message Chain

Code traverses the object graph, sending messages not only to its immediate collaborators but to the collaborators of those collaborators.

Also known as a "Train Wreck" or "Law of Demeter Violation".

What It Looks Like

def shipment_delivered?
  !shipment.delivery_date.to_s.strip.empty?
end

Why It Hurts

Dependencies on transitive collaborators make code brittle. Changes to any object involved in the message chain can break the code that's sending the messages. In addition, it's painful to mock transitive collaborators in unit tests.

How To Fix It

Similar to Feature Envy: Use Move Method and Extract Method. If you have deeply nested data objects, you may have to flatten a Deep Hierarchy.

Caveats

The Builder Pattern results in message chains, but these are not harmful; since each method returns self, you're always talking to the same object.