避免在聚合中引用实体的最佳方法是什么？

Question

I am new to DDD. I just started a project by DDD architecture. In domain design I faced a problem. Given I have a customer that supposed to send me a list of his/her orders, for example:

customer1:{Onion,Apple,orange,wireless Mouse,PC,fish,egg}

every item can be delegated to different store:

Grocery Store:{onion,apple,orange,fish,egg}
Digital Store:{wireless mouse,PC}
butchery:{fish,egg}

every order just have a name. to validate list of orders for customer, I should delegate validation to customer as an 'AggregateRoot' and consider list of orders as 'List':

public class Customer : AggregateRoot<long>
{
    public long Id { get;private set; }
    public string CustomerName { get;private set; }

    private List<Order> _orders = new List<Order>();
    public IReadOnlyCollection<Order> Orders => _orders.AsReadOnly();

    public Customer(long id, string customerName)
    {
        Id = id;
        CustomerName = customerName;
    }

    public void AssignOrders(List<Order> orders)
    {
        //check some validation on orders before assignments
        this._orders=orders;
    }
}

public class Order : Entity<long>
{
    public long Id { get;private set; }
    public string Name { get;private set; }

    public Order(long id,string name)
    {
        //check some validation for single order before creation
        this.Id=id;
        this.Name=name; 
    }
}

but my design problem arises when i want assign 'orders' to 'stores': based on DDD Rule "An aggregate root should not hold references to an entity in another aggregate root"

public class Store:AggregateRoot<long>
{
    public long Id { get;private set; }
    public string StoreName { get;private set; }
    private List<Order> _orders = new List<Order>();
    public IReadOnlyCollection<Order> Orders => _orders.AsReadOnly();

    public Store(string storeName, long id)
    {
        StoreName = storeName;
        Id = id;
    }

    public void AssignOrders(List<Order> orders)
    {
        //check some validation on orders before assignments
        this._orders = orders;
    }
}

In My design 'Store' is an aggregate root and needs to have list of orders. Beside there are orders assigned to multiple stores. now what is your suggestion:

1. Should I keep 'orders' as entity in 'Customer' and assigns a list of 'Store' to every single 'Order'.
2. Should I Keep 'Orders' as an Aggregate root with a list of orders and a 'customerId', but then how to validate list altogether?
3. Should I change 'Orders' in 'customer' to list of value objects and just copy them for orders (Hard to maintain, Cause there are other agregate roots that need a list of 'orders', and store is one of them)

Answer 1

I suggest you talk to your domain experts to clarify how the business is done, if you can. Then you can find distinct entities that you need to model.

It looks like you may have 2 subdomains ("world views") here: Customer and Store which will both have a concept of an Order , but those orders are different.

Possibly this model can be of use:

CustomerOrder - a set of items that customer wants to buy. It has a number of Customer Order Line objects which contain Product ID / Quanity / Volume . When you create this CustomerOrder object you can validate it. You can also validate each Customer Order Line with some simple checks, for example, that at least volume or quantity > 0 etc.

StoreOrder is the order that store receives. It also contains a list of Store Order Line objects. It is different thing to the CustomerOrder . It will contain different items, may be aggregated, can have additional information in it etc.

Talk to your domain experts to validate the model. If you don't have any domain experts handy, implement something that will get you going and evolve it when needed.