Entities

Entities are one of the core concepts of DDD (Domain Driven Design). Eric Evans describe it as "An object that is not fundamentally defined by its attributes, but rather by a thread of continuity and identity".

An entity is generally mapped to a table in a relational database.

Entity Class

Entities are derived from the Entity<TKey> class as shown below:

public class Book : Entity<Guid>
{
    public string Name { get; set; }

    public float Price { get; set; }
}

If you do not want to derive your entity from the base Entity<TKey> class, you can directly implement IEntity<TKey> interface.

Entity<TKey> class just defines an Id property with the given primary key type, which is Guid in the example above. It can be other types like string, int, long or whatever you need.

Entities with GUID Keys

If your entity's Id type is Guid, there are some good practices to implement:

• Create a constructor that gets the Id as a parameter and passes to the base class.
  • If you don't set a GUID Id, ABP Framework sets it on save, but it is good to have a valid Id on the entity even before saving it to the database.
• If you create an entity with a constructor that takes parameters, also create a protected empty constructor. This is used while your database provider reads your entity from the database (on deserialization).
• Don't use the Guid.NewGuid() to set the Id! Use the IGuidGenerator service while passing the Id from the code that creates the entity. IGuidGenerator optimized to generate sequential GUIDs, which is critical for clustered indexes in the relational databases.

An example entity:

public class Book : Entity<Guid>
{
    public string Name { get; set; }

    public float Price { get; set; }

    protected Book()
    {

    }

    public Book(Guid id)
     : base(id)
    {

    }
}

Example usage in an application service:

public class BookAppService : ApplicationService, IBookAppService
{
    private readonly IRepository<Book> _bookRepository;

    public BookAppService(IRepository<Book> bookRepository)
    {
        _bookRepository = bookRepository;
    }

    public async Task CreateAsync(CreateBookDto input)
    {
        await _bookRepository.InsertAsync(
            new Book(GuidGenerator.Create())
            {
                Name = input.Name,
                Price = input.Price
            }
        );
    }
}

• BookAppService injects the default repository for the book entity and uses its InsertAsync method to insert a Book to the database.
• GuidGenerator is type of IGuidGenerator which is a property defined in the ApplicationService base class. ABP defines such frequently used base properties as pre-injected for you, so you don't need to manually inject them.
• If you want to follow the DDD best practices, see the Aggregate Example section below.

Entities with Composite Keys

Some entities may need to have composite keys. In that case, you can derive your entity from the non-generic Entity class. Example:

public class UserRole : Entity
{
    public Guid UserId { get; set; }

    public Guid RoleId { get; set; }
    
    public DateTime CreationTime { get; set; }

    public UserRole()
    {
            
    }
    
    public override object[] GetKeys()
    {
        return new object[] { UserId, RoleId };
    }
}

For the example above, the composite key is composed of UserId and RoleId. For a relational database, it is the composite primary key of the related table. Entities with composite keys should implement the GetKeys() method as shown above.

Notice that you also need to define keys of the entity in your object-relational mapping (ORM) configuration. See the Entity Framework Core integration document for example.

Also note that Entities with Composite Primary Keys cannot utilize the IRepository<TEntity, TKey> interface since it requires a single Id property. However, you can always use IRepository<TEntity>. See repositories documentation for more.

AggregateRoot Class

"Aggregate is a pattern in Domain-Driven Design. A DDD aggregate is a cluster of domain objects that can be treated as a single unit. An example may be an order and its line-items, these will be separate objects, but it's useful to treat the order (together with its line items) as a single aggregate." (see the full description)

AggregateRoot<TKey> class extends the Entity<TKey> class. So, it also has an Id property by default.

Notice that ABP creates default repositories only for aggregate roots by default. However, it's possible to include all entities. See the repositories documentation for more.

ABP does not force you to use aggregate roots, you can in fact use the Entity class as defined before. However, if you want to implement the Domain Driven Design and want to create aggregate root classes, there are some best practices you may want to consider:

• An aggregate root is responsible to preserve it's own integrity. This is also true for all entities, but aggregate root has responsibility for it's sub entities too. So, the aggregate root must always be in a valid state.
• An aggregate root can be referenced by it's Id. Do not reference it by it's navigation property.
• An aggregate root is treated as a single unit. It's retrieved and updated as a single unit. It's generally considered as a transaction boundary.
• Work with sub-entities over the aggregate root- do not modify them independently.

See the entity design best practice guide if you want to implement DDD in your application.

Aggregate Example

This is a full sample of an aggregate root with a related sub-entity collection:

public class Order : AggregateRoot<Guid>
{
    public virtual string ReferenceNo { get; protected set; }

    public virtual int TotalItemCount { get; protected set; }

    public virtual DateTime CreationTime { get; protected set; }

    public virtual List<OrderLine> OrderLines { get; protected set; }

    protected Order()
    {

    }

    public Order(Guid id, string referenceNo)
    {
        Check.NotNull(referenceNo, nameof(referenceNo));
        
        Id = id;
        ReferenceNo = referenceNo;
        
        OrderLines = new List<OrderLine>();
    }

    public void AddProduct(Guid productId, int count)
    {
        if (count <= 0)
        {
            throw new ArgumentException(
                "You can not add zero or negative count of products!",
                nameof(count)
            );
        }

        var existingLine = OrderLines.FirstOrDefault(ol => ol.ProductId == productId);

        if (existingLine == null)
        {
            OrderLines.Add(new OrderLine(this.Id, productId, count));
        }
        else
        {
            existingLine.ChangeCount(existingLine.Count + count);
        }

        TotalItemCount += count;
    }
}

public class OrderLine : Entity
{
    public virtual Guid OrderId { get; protected set; }

    public virtual Guid ProductId { get; protected set; }

    public virtual int Count { get; protected set; }

    protected OrderLine()
    {

    }

    internal OrderLine(Guid orderId, Guid productId, int count)
    {
        OrderId = orderId;
        ProductId = productId;
        Count = count;
    }

    internal void ChangeCount(int newCount)
    {
        Count = newCount;
    }
    
    public override object[] GetKeys()
    {
        return new Object[] {OrderId, ProductId};
    }
}

If you do not want to derive your aggregate root from the base AggregateRoot<TKey> class, you can directly implement the IAggregateRoot<TKey> interface.

Order is an aggregate root with Guid type Id property. It has a collection of OrderLine entities. OrderLine is another entity with a composite primary key (OrderId and ProductId).

While this example may not implement all the best practices of an aggregate root, it still follows some good practices:

• Order has a public constructor that takes minimal requirements to construct an Order instance. So, it's not possible to create an order without an id and reference number. The protected/private constructor is only necessary to deserialize the object while reading from a data source.
• OrderLine constructor is internal, so it is only allowed to be created by the domain layer. It's used inside of the Order.AddProduct method.
• Order.AddProduct implements the business rule to add a product to an order.
• All properties have protected setters. This is to prevent the entity from arbitrary changes from outside of the entity. For exmple, it would be dangerous to set TotalItemCount without adding a new product to the order. It's value is maintained by the AddProduct method.

ABP Framework does not force you to apply any DDD rule or patterns. However, it tries to make it possible and easier when you do want to apply them. The documentation also follows the same principle.

Aggregate Roots with Composite Keys

While it's not common (and not suggested) for aggregate roots, it is in fact possible to define composite keys in the same way as defined for the mentioned entities above. Use non-generic AggregateRoot base class in that case.

Base Classes & Interfaces for Audit Properties

There are some properties like CreationTime, CreatorId, LastModificationTime... which are very common in all applications. ABP Framework provides some interfaces and base classes to standardize these properties and also sets their values automatically.

Auditing Interfaces

There are a lot of auditing interfaces, so you can implement the one that you need.

While you can manually implement these interfaces, you can use the base classes defined in the next section to simplify it.

• IHasCreationTime defines the following properties:
  • CreationTime
• IMayHaveCreator defines the following properties:
  • CreatorId
• ICreationAuditedObject inherits from the IHasCreationTime and the IMayHaveCreator, so it defines the following properties:
  • CreationTime
  • CreatorId
• IHasModificationTime defines the following properties:
  • LastModificationTime
• IModificationAuditedObject extends the IHasModificationTime and adds the LastModifierId property. So, it defines the following properties:
  • LastModificationTime
  • LastModifierId
• IAuditedObject extends the ICreationAuditedObject and the IModificationAuditedObject, so it defines the following properties:
  • CreationTime
  • CreatorId
  • LastModificationTime
  • LastModifierId
• ISoftDelete (see the data filtering document) defines the following properties:
  • IsDeleted
• IHasDeletionTime extends the ISoftDelete and adds the DeletionTime property. So, it defines the following properties:
  • IsDeleted
  • DeletionTime
• IDeletionAuditedObject extends the IHasDeletionTime and adds the DeleterId property. So, it defines the following properties:
  • IsDeleted
  • DeletionTime
  • DeleterId
• IFullAuditedObject inherits from the IAuditedObject and the IDeletionAuditedObject, so it defines the following properties:
  • CreationTime
  • CreatorId
  • LastModificationTime
  • LastModifierId
  • IsDeleted
  • DeletionTime
  • DeleterId

Once you implement any of the interfaces, or derive from a class defined in the next section, ABP Framework automatically manages these properties wherever possible.

Implementing ISoftDelete, IDeletionAuditedObject or IFullAuditedObject makes your entity soft-delete. See the data filtering document to learn about the soft-delete pattern.

Auditing Base Classes

While you can manually implement any of the interfaces defined above, it is suggested to inherit from the base classes defined here:

• CreationAuditedEntity<TKey> and CreationAuditedAggregateRoot<TKey> implement the ICreationAuditedObject interface.
• AuditedEntity<TKey> and AuditedAggregateRoot<TKey> implement the IAuditedObject interface.
• FullAuditedEntity<TKey> and FullAuditedAggregateRoot<TKey> implement the IFullAuditedObject interface.

All these base classes also have non-generic versions to take AuditedEntity and FullAuditedAggregateRoot to support the composite primary keys.

All these base classes also have ...WithUser pairs, like FullAuditedAggregateRootWithUser<TUser> andFullAuditedAggregateRootWithUser<TKey, TUser>. This makes possible to add a navigation property to your user entity. However, it is not a good practice to add navigation properties between aggregate roots, so this usage is not suggested (unless you are using an ORM, like EF Core, that well supports this scenario and you really need it - otherwise remember that this approach doesn't work for NoSQL databases like MongoDB where you must truly implement the aggregate pattern).

See Also

• Best practice guide to design the entities