In general, I am tempted to answer Mu (in the sense of Zen), because the script is incorrect in terms of DDD. In DDD, we start with the business requirements and domain experts, and from the ones that we get the Domain Model .
Although this is a controversial point, a database is just a random artifact of business requirements (almost always indicating that entities should be preserved).
Nevertheless, I will try to do my best.
In most cases, ordering is a fairly important business object, and obviously we need to know about order lines, including products on each line, so we think we need an association from the order line (Order_Detail) for the Product.
However, when a given specific product we rarely need to know in which orders it was included, so it offers one-way relationships right there. We can move from the order line to the product, but not from the product line to the order.
However, the above analysis may turn out to be false at a deeper level. Imagine the following conversation between a developer and a domain expert:
Dev: We created this association from orders to products so that we always know everything about products in a specific order.
Exp: That sounds good, but what about the provider?
Dev: This is also included.
Exp: So what happens when we change the supplier for a product?
Dev: This will be implicitly reflected in the order data ...
Exp: This is not what we want. We want the data to reflect the order at the time of delivery.
In this case, it turns out that there is an error in the database schema. The problem may be caused by messages, as business analysts may want to run reports on how different suppliers affect profits (which I know).
Such a case may suggest trimming the association from order lines to the product as a whole. Orders must contain historical data (snapshots), not current product data.
We can even introduce a ProductSnapshot Value object that semantically reflects Product Entity to model this in a domain model.
In general, it seems more reasonable to model Order as a combination of itself and order lines (with ProductSnapShots ), but what about the relationship between orders and customers?
Since I currently understand associations and aggregates, associations define aggregates . Given the order, would we like to know about the client? Likely. Given the customer, do you want to know about orders? Maybe.
This implies a two-way communication that makes Customer Aggregate Root . This means that you will have a CustomerRepository , but not an OrderRepository . Every time you need an order, you must receive it through Customer .
In some cases, this can make sense, while it can be really awkward in other situations. It really depends on business requirements ...
You might also want to create an OrderRepository , but this invades the Customer Aggregate Root. If you do this, it becomes uncertain where the responsibility for the Order lies. What happens if you draw from an order to a customer? Customer has a list of orders, but all of them are filled in memory, if you read Order from OrderRepository ?
Probably not, but they will probably be if you read the Customer from CustomerRepository . The point here is that the analysis of Aggregate Roots is important, and once you have identified Aggregate, you will have to stick to it and respect it.
This forces me to maintain small aggregates over large aggregates, so in this example I would hold back the aggregate until Order and its order lines and had no connection between Order and Customer .
The lack of a formal relationship between Order and Customer does not mean that we cannot link them at all, but we need explicit services to give us all Orders for this Client. We cannot just move from one to another.