Using Aggregation and Association

Question

Say I have two classes: Customer and Invoice. The database tables look like this:

CREATE TABLE Customer (ID int, name varchar(30), primary key (ID))
CREATE TABLE Invoice (ID int, date datetime, foreign key (CustomerID) references Customer(ID), primary key (ID))

I am trying to decide whether this relationship would be modelled using Aggregation or Association:

Aggregation

public class Customer
{
 int ID;
 string Name;
 List<Invoice> Invoices;
 public void DoSomething()
 {
 //Do something with Customer.Invoices
 }
}

Here the customer object (including Invoices) is populated from the ORM.

Association

public class Customer
{
 int ID;
 string Name;
 public Customer int id, string name, list<Invoice> invoices)
 {
 ID=id;
 Name = name;
 Invoices=invoices;
 }
 public void DoSomething(List<Invoice> Invoices)
 {
 //Do something with local Invoices
 }
 }

Say I have two user stories. One user story requires the Invoices to outlive the Customer and one user story requires the Invoice to be deleted with the Customer. How would I model this? Would I have n Invoices member (like code fragment one) and a method that accepts Invoices as an argument (like code fragment two)?

I am not developing a system that uses Customers and Invoices. I am just trying to improve my thinking.

• Stop harming Monica: The answer depends on the concrete requirements of the application you are not developing.
• Greg Burghardt: If an invoice can exist without a customer then the CustomerId column is NULLABLE. This is more a problem for the data mapping, not a C# class where objects are in managed memory. This would change for a language like C++ where the memory is unmanaged.

Answer 1

It has to do with how you feel about delete.

In Aggregation, both the entries can survive individually which means ending one entity will not effect the other entity

When there is a composition between two entities, the composed object cannot exist without the other entity.

geeksforgeeks.org: Association Composition Aggregation

So if the legal team stormed the IT department with a court order and forced you to delete a customer record do you want to automatically delete the customers invoices as well, even if that's not part of the court order?

There are many reasons to delete. It's the meaning of the delete that dictates how it should cascade through the system. How will the system behave if invoices point to customers that don't exist? Is that how you want it to behave?

• w0051977: Thanks. Is there ever a scenario where you would exclude navigation properties from the domain model? For example, you have a 'Customer' and 'Invoice' table in the database, however in the domain layer Customer does not include an Invoices collection and Invoice does not include a Customer object?
• candied_orange: Sure, there are things you do with customers that have nothing to do with invoices, like say a cold call sheet. Don't feel like you're only allowed to have one Customer class that does everything that touches the Customer table. Just use nice descriptive names so people can find stuff.
• w0051977: Thanks. I guess you could have a table called CustomerInvoices and CustomerColdCallSheet etc?
• candied_orange: Exactly. There is a lot of flexibility to be gained if you don't write your code as if it has to parrot what's in the database or vise versa. Write the code you need. No more. No less.
• w0051977: Is it fair to say that if you have two classes that are many to many then you must use an Association rather than an Aggregate or Composition? I realise I have accepted the answer, however I can still upvote it.