What am I missing with the Respository Pattern? How do you use it in the real world?_问答_开发者

Every example of the Repository Pattern I have seen deals with a very simple use case - one object type and the most basic CRUD operations. The Repository is then very often plugged straight into an MVC controller.

Real-world data access just isn't like this. Real-world data access scenarios can in开发者_JAVA技巧volve complex graphs of objects and some form of transactional wrapper. For example, suppose I want to save a new Order. This involves writing to the Order, OrderDetails, Invoice, User, History and ItemStock tables. All of this must be transacted, committed or rolled back. Normally I'd pass around something like an IDbTransaction and an IDbConnection and bundles the whole operation in a service layer.

Where does the Repository Pattern fit with this? Am I missing something (Unit Of Work perhaps)? Are there any more realistic examples of Repositories in use than the usual canned blog snippets?

Appreciate any light.

This is a very controversial subject but here is my catch from my own experience.

Repository works at the aggregate root. For example if an OrderItem is always retrieved as part of Order and does not have a life of its own outside order, then it will be loaded by the OrderRepository, otherwise it will have its own repository.

UnitOfWork is an important concept. Let's say OrderItem is an aggregate root and has its own repository. So at the time of creating an order, OrderManager will create a UnitOfWork of work in a using block, initialise OrderItemRepository and OrderRepository and then commit.

UPDATE

Yes, exactly. Imagine - in our case order - an order is being inserted. This needs to be in control of the transaction and enter order and order items separately inside the same transaction. This cannot be managed at the repository level. This is the sole reason for existence of UnitOfWork concept which is passed to the repository so that it does not own or initialise it. UnitOfWork usually is created at the business layer.

O/R-Mappers like Hibernate basically implement the Repository pattern for object graphs while fully supporting transactions. It's often a leaky abstraction, but it certainly can be made to work in complex real-world scenarios.

If you need a good full blown, widely used expample of the repository pattern look at Cocoa's Core Data I realize it is not in the realm of programming languages that you note. But note that it is NOT an O/R mapper. It is a complete abstraction of an object store. No Sql statements to execute, and while you may pick the format of the external storage that is used, you never interact with it directly.

I like to think of a repository as another layer of abstraction. You should only add layers of abstraction when the cost of their implementation is less than the cost of NOT doing the implementation (code maintenance, support, enhancements, etc).

Remember that the purpose of the repository pattern is to separate the logic that retrieves the data (CRUD) and maps it to the entity model from the business logic that acts on the model. What this often ends up doing/looking like in the real world is some form of business entities, that abstract the underlying physical data model.

As far as your transaction question, yes, this relates more to the Unit of Work pattern. Since you mentioned services, I would encourage you to NOT pass around your connection to your various data access classes/methods, but to instead allow WCF to manage the transaction for you using auto enlistment. Here is an extract of Juval Lowy's WCF book (highly recommended) that explains the how and why of this method of transaction management.

So to answer your question, the repository pattern fits in as a way to abstract the physical data model and to separate the CRUD/mapping from the business logic.