I generally try and keep all related entities in the same repository. The following are entities that have a relationship between the two (marked with indentation):
- User
- UserPreference
So they make sense to go into a user repository. However users are often linked to many different entities, what would you do in the following example?
-
User
- UserPrefence
- Order
-
Order
- Product
Order has a relationship with both product and user but you wouldn’t put functionality for all 4 entities in the same repository. What do you do when you are dealing with the user entities and gathering order information? You may need extra information about the product and often ORMs will offer the ability of lazy loading. However if your product entity is in a separate repository to the user entity then surely this would cause a conflict between repositories?
In the Eric Evan’s Domain Driven Design ( http://domaindrivendesign.org/index.htm ) sense of things you should first think about what about your Aggregates. You then build you repositories around those.
There are many techniques for handling Aggregates that relate to each other. The one that I use most often is to only allow Aggregates to relate to each other through a read only interface. One of the key thoughts behind Aggregates is that you can’t change state of underlying objects without going through the root. So if Product and User are root Aggregates in your model than I can’t update a Product if I got to it by going through User->Order->Product. I have to get the Product from the Product repository to edit it. (From a UI point of view you can make it look like you go User->Order->Product, but when you hit the Product edit screen you grab the entity from the Product Repository).
When you are looking at a Product (in code) by going from User->Order->Product you should be looking at a Product interface that does not have any way to change the underlying state of the Product (only gets no sets etc.)
Organize your Aggregates and therefor Repositories by how you use them. I can see User and Prodcut being their own Aggregates and having their own Repositories. I’m not sure from your description if Order should belong to User or also be stand alone.
Either way use a readonly interface when Aggregates relate. When you have to cross over from one Aggregate to the other go fetch it from its own Repository.
If your Repositories are caching then when you load an Order (through a User) only load the Product Id’s from the database. Then load the details from the Product Repository using the Product Id. You can optimize a bit by loading any other invariants on the Product as you load the Order.