This post advocates for using Ruby's built-in features like Struct and immutable data structures (via freeze) to create simple, efficient value objects. It argues against using more complex approaches like dry-struct or Virtus for basic cases, highlighting that the lightweight, idiomatic approach often provides sufficient functionality with minimal overhead. The article illustrates how Struct provides concise syntax for defining attributes and automatic equality and hashing based on those attributes, fulfilling the core requirements of value objects. Finally, it demonstrates how to enforce immutability by freezing instances, ensuring predictable behavior and preventing unintended side effects.
This blog post elucidates the creation of value objects in Ruby, emphasizing the idiomatic approach favored by experienced Ruby developers. It begins by defining what constitutes a value object: an immutable object whose identity is determined solely by its attributes. This means two value objects with the same attribute values are considered equal, regardless of their memory location. The author contrasts this with entity objects, which maintain individual identities even with identical attribute values.
The post then delves into the preferred Ruby method for crafting value objects, leveraging the Struct class. Struct provides a concise and efficient mechanism for defining immutable data structures with automatically generated accessor methods. The author demonstrates how to create a simple Point value object using Struct, highlighting the automatic inclusion of methods like #== and #hash which correctly compare objects based on attribute values, fulfilling the core requirements of a value object.
Furthermore, the post showcases how to incorporate custom methods within Struct-based value objects. This extends their functionality beyond mere data storage. The author uses an example of adding a distance method to the Point object, demonstrating how to encapsulate relevant logic within the value object itself. This exemplifies the power of Struct to create not just data containers, but genuinely useful and self-contained objects.
The author stresses the importance of immutability for value objects and demonstrates how to enforce it using the #freeze method. Freezing a Struct object prevents any subsequent modification of its attributes, ensuring that its state remains constant throughout its lifecycle, reinforcing its value object nature. The post specifically warns against using OpenStruct for value objects due to its inherent mutability.
Finally, the post briefly touches upon alternative approaches for creating value objects, including using classes and defining methods manually. However, it reiterates the advantages of the Struct-based approach, highlighting its conciseness, readability, and automatic generation of crucial comparison methods, concluding that Struct is the most idiomatic and therefore preferred way to implement value objects in Ruby. This conciseness minimizes boilerplate code and promotes clarity, aligning with the Ruby philosophy of elegant and expressive code. The post ultimately champions the Struct class as the most effective and Ruby-like solution for creating value objects.
Summary of Comments ( 1 )
https://news.ycombinator.com/item?id=43433648
HN users largely criticized the article for misusing or misunderstanding the term "Value Object." Commenters pointed out that true Value Objects are immutable and compared by value, not identity. They argued that the article's examples, particularly using mutable hashes and relying on
equal?, were not representative of Value Objects and promoted bad practices. Several users suggested alternative approaches like usingStructor creating immutable classes with custom equality methods. The discussion also touched on the performance implications of immutable objects in Ruby and the nuances of defining equality for more complex objects. Some commenters felt the title was misleading, promoting a non-idiomatic approach.The Hacker News post titled "How to create value objects in Ruby – the idiomatic way" (linking to an article about creating value objects in Ruby) has several comments discussing various aspects of value objects, their implementation in Ruby, and alternative approaches.
One commenter points out the inherent tension between true value object semantics (immutable, compared by value) and the performance implications of creating new objects for every modification. They highlight the practical compromise often made in Ruby where objects are treated as if they were value objects, even if they are technically mutable under the hood. This commenter also raises the question of whether the performance cost of true immutability is actually significant in typical Ruby applications.
Another commenter emphasizes the importance of clearly defining equality (
==) and hash code (hash) methods when working with value objects in Ruby. They mention that usingStructcan simplify this process, but caution against overlooking these crucial methods for correct value object behavior.The discussion then delves into specific aspects of Ruby's object model and how it affects value object implementation. One commenter argues against using
dupfor creating modified copies of value objects, preferring explicit constructor calls or factory methods for clarity and control. They also advocate for defining methods that return new instances rather than modifying the existing object in place. Another commenter suggests leveraging thedry-structgem, which provides built-in support for immutability and value comparisons. This suggestion sparks a brief comparison ofdry-structwithValueandData.define, two other Ruby gems designed for creating value objects, highlighting the tradeoffs between different approaches.A separate thread within the comments discusses the use of
freezefor enforcing immutability in Ruby. One commenter cautions against overusingfreeze, particularly when dealing with nested data structures. They explain thatfreezeonly provides shallow immutability, leaving deeper layers potentially mutable, which can lead to unexpected behavior.Finally, a few comments touch on the broader context of value objects and their relationship to domain-driven design (DDD). They suggest that focusing on the conceptual aspects of value objects, namely their role in representing domain concepts, is more important than the specific implementation details. One commenter highlights the importance of understanding the business logic and how value objects contribute to the overall domain model.