Composition in OpenEdge classes

I have a question regarding the use of composition in OpenEdge classes. 

I come from a C# / Java background and it looks like OpenEdge takes a different approach here. 

We have a use case where an order should be added to the database. 

In C# (using Entity Framework) I would create some entities like Order, OrderLine and Customer

The way the entity classes would be build is like this:

class Order {

    public int OrderId {get; set; }

    public int CustomerId {get; set; }

    public virtual Customer Customer { get; set; }

    public virtual ICollection<OrderLine> OrderLines {get; set; }

}

class Customer {

    public int CustomerId {get; set; }

    public virtual ICollection<Order> Orders { get; set; }

}

class OrderLine {

    public int OrderLineId {get; set; }

    public int OrderId { get; set; }

    public virtual Order Order {get; set; }

}

I have done several OpenEdge courses but am still trying to get all the pieces into the right place here.

Like in the IntroToOOP course I can see that I would have to use an order temp-table with a CustomerObj field. And the Customer object would hold a temp-table containing all the customer info. But then what would I do with all the properties for Customer.

When a property updates, that that would have to be changed in the Customer object and in the Customer temp-table.

What I would like to do is just use a  AddOrder() method in the Customer class to create a new order, and then do something like customer:order:AddOrderLine().

What would be the best way to accomplish this in OpenEdge?