One Into Many

Overview

This pattern is to be used in cases where a previous implementation has a one to one relationship and it needs to be modified to a one to many relationship. This pattern is specifically designed for Objects; however it may be used for lower or higher level concepts.

The Problem

During initial coding/design, you failed to notice that a relationship had the potential to be one-to-many and instead coded it as one-to-one. Now, you realise your problem and need to rectify it.

Solution

1. If the class/object implements a Singleton pattern, remove the code required to save a single instance.
   1. Write a test to make sure multiple instances of the object can be instantiated, i.e. simply creating multiple instances.
   2. Change the code to work by changing the constructor access.
   3. Remove unnecessary code.
   4. Refactor as necessary.
2. If it is a simple case of an object only being held by another object once when multiple are required, for instance, in a model, use a collection like a Set or List to encapsulate these objects.
   1. Do not change existing interface initially.
   2. Write a test for the existence of the collection.
   3. Replace the existing field with a field holding the collection, making sure to rename the field.
   4. Get the test to past.
   5. Refactor to adjust any constructors as necessary.
   6. Refactor to change any existing getter/setter methods if required.
   7. Make sure all holding classes work for the situation.
   8. Make sure all aggregated classes work for the situation.
   9. Check any classes that use the model, refactor as necessary.
3. If a more complex problem exists, follow these steps:
   1. ...