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 Entity<TKey> class as shown below:

public class Person : Entity<int>
{
    public string Name { get; set; }

    public DateTime CreationTime { get; set; }

    public Person()
    {
        CreationTime = DateTime.Now;
    }
}

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 int in the sample above. It can be other types like string, Guid, long or whatever you need.

Entity class also overrides the equality operator (==) to easily check if two entities are equal (they are equals if they are same entity type and their Ids are equals).

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.

Composite primary keys has a restriction with repositories. Since it has not known Id property, you can not use IRepository<TEntity, TKey> for these entities. 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 class extends the Entity 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 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 DDD 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.

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;
    }
}

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 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 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.