Entity or Value Object: that is the question
In Domain-Driven Design (DDD), one of the key decisions you’ll encounter is whether a particular concept should be modeled as an Entity or a Value Object. This decision can significantly impact the design and behavior of your software.
In this blog post, we’ll explore the distinction between Entities and Value Objects and provide a method to help you determine which is the right choice for a given domain concept.
Understanding Entities and Value Objects
Before diving into the decision-making process, let’s clarify what Entities and Value Objects are in the context of DDD.
Entities
Entities represent objects with a distinct identity that runs throughout their entire lifecycle. They are characterized by their unique identifiers, such as an ID or a combination of attributes that make them distinguishable.
An Entity’s equality is based on its identity, not its attributes. In other words, two entities with the same attributes are still considered different if they have different identities.
When an object is distinguished by its identity, rather than its attributes, make this primary to its definition in the model. Keep the class definition simple and focused on life cycle continuity and identity.
Entities often correspond to real-world objects that you want to track and reference independently. For example, a “Customer” in an e-commerce system is typically an Entity because each customer has a unique ID, and you need to distinguish them even if their attributes (name, email, etc.) are the same.
Value Objects
Value Objects, on the other hand, are objects without distinct identities. They are defined solely by their attributes, and their equality is based on the equality of those attributes. Value Objects represent concepts that are immutable, and any change results in a new Value Object instance.
When you care only about the attributes and logic of an element of the model, classify it as a value object. Make it express the meaning of the attributes it conveys and give it related functionality.
Value Objects are often used to represent things like dates, money, addresses, or measurements. For instance, a “DateOfBirth” object is a Value Object because it’s defined by its date value, and two DateOfBirth objects with the same date are considered equal, regardless of where or how they are used.
The Decision-Making Method
Now that we have a clear understanding of Entities and Value Objects, let’s explore a method for making the right choice when modeling concepts in your domain.
Step 1: Identify the Concept
Start by identifying the concept you want to model. Ask yourself whether it has a distinct identity or if it’s solely defined by its attributes.
Step 2: Determine Identity
If the concept has a distinct identity that remains consistent throughout its lifecycle, it’s likely an Entity. Consider whether you need to track, reference, or relate this concept individually. If so, an Entity is the right choice.
Step 3: Consider Mutability
Think about whether the concept is immutable or can change over time. If it’s immutable and its equality is based solely on its attributes, it’s a strong indicator that it should be a Value Object.
Step 4: Analyze Use Cases
Consider how the concept will be used in your domain. If you often need to perform operations or comparisons based on its identity, it’s likely an Entity. If it’s frequently used as a set of attributes and equality comparisons are attribute-based, it’s a Value Object.
Step 5: Consult Domain Experts
Always involve domain experts and stakeholders in the decision-making process. Their insights can help clarify the nature of the concept and guide your modeling choices.
Step 6: Refine as Needed
It’s essential to iterate and refine your modeling decisions as your understanding of the domain evolves. Don’t be afraid to adjust your model based on real-world usage and feedback.
Conclusion
The distinction between Entities and Value Objects in DDD is crucial for building a domain model that accurately represents your problem space.
By following the decision-making method outlined above, you can make informed choices when modeling concepts and ensure that your software aligns with the domain’s inherent structure. Remember that the ultimate goal is to create a domain model that is both expressive and robust, enabling you to build more maintainable and effective software.
If you need more details about this topic, useful resources can be found here.
I hope that this article was useful for you! 👋
