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If you have been watching this blog, you know that I have been discussing the various aspects of POCO capabilities we added to Entity Framework 4.0. POCO support makes it possible to do persistence ignorance with Entity Framework in a way that was never possible with Entity Framework 3.5.
If you missed the series on POCO, I’ve listed them here for your convenience. It might be a good idea to quickly check these out.
In this post, I’d like to look at how we might be able to take our example a bit further and use some of the common patterns such as Repository and Unit Of Work so that we can implement persistence specific concerns in our example.
Expanding on our Northwind based example a bit further, let’s say I am interested in supporting the following Customer entity oriented operations:
- Query for a customer by ID
- Searching for Customer by Name
- Adding a new Customer to the database
I also want to be able to query for a product based on ID.
Finally, given a customer, I would like to be able to add an Order to the database
Before we get into the details, there are two things I’d like to get out of the way first:
- There is more than one “correct” way to approach this problem. I’ll likely be making many simplifying assumptions as I go – the objective is to show a very high level sketch of how you might implement the two patterns to solve the problem at hand.
- Normally, when using TDD, I’d start out with tests and use my tests to evolve my design incrementally. Since I’m not following TDD in this example, please bear with me if you see me doing things like defining an interface upfront, instead of letting tests and commonalities dictate the need for an interface, etc.
Implementing the Repository
Let’s start with the work we have to do on Customer entity and look at what a repository for dealing with Customer might look like:
This repository interface seems to meet all the requirements around Customer:
- GetCustomerById should allow me to get a single customer entity by primary key
- FindByName should allow me to search for customers
- AddCustomer should allow me to add customer to the database
This sounds good to me for the moment. Defining an interface like this for your repository is a good idea, especially if you are interested in writing tests using mocks or fakes and it allows for better unit testing by keeping your database out of the equation entirely. There are blog posts coming in the future that cover testability, mocks and fakes, etc.
You might take this interface definition a bit further and define a common IRepository for dealing with concerns that are common for multiple repository types. This is fine thing to do if you see that it works for you. I don’t necessarily see the need yet for this particular example and so I’ll pass on it for now. It is entirely possible that this becomes important as you add more repositories and refactor.
Let’s take this repository and see how we might build an implementation of it that leverages Entity Framework to enable data access.
First of all, I need an ObjectContext that I can use to query for data. You might be tempted to handle ObjectContext instantiation as a part of the repository’s constructor – but it is a good idea to leave that concern out of the repository and deal with that elsewhere.
Here’s my constructor:
In the above snippet, NorthwindContext is my typed ObjectContext type.
Let’s now provide implementations for the methods required by our ICustomerRepository interface.
GetCustomerById is trivial to implement, thanks to LINQ. Using standard LINQ operators, we can implement GetCustomerById like this:
Similarly, FindByName could look like this. Once again, LINQ support makes this trivial to implement:
Note that I chose to expose the results as IEnumerable<T> – you might choose to expose this as an IQueryable<T> instead. There are implications to doing this – in this case, I am not interested in exposing additional IQueryable based query composition over what I return from my repository.
And finally, let’s see how we might implement AddCustomer:
You may be tempted to also implement the save functionality as a part of the AddCustomer method. While that may work for this simple example, it is generally a bad idea – this is exactly where the Unit of Work comes in and we’ll see in a bit how we can use the this pattern to allow us to implement and coordinate Save behavior.
Here’s the complete implementation of CustomerRepository that uses Entity Framework for handling persistence:
Here’s how we might use the repository from client code:
For dealing with my Product and Order related requirements, I could define the following interfaces (and build implementations much like CustomerRepository). I’ll leave the details out of this post for brevity.
Implementing Unit of Work Pattern using ObjectContext
You may have noticed this already; even though we didn’t implement any specific pattern to explicitly allow us to group related operations into a unit of work, we are already getting Unit of Work functionality for free with NorthwindContext (our typed ObjectContext).
The idea is that I can use the Unit of Work to group a set of related operations – the Unit of Work keeps track of the changes that I am interested in until I am ready to save them to the database. Eventually, when I am ready to save, I can do that.
I can define an interface like this to define a “Unit of Work”:
Note that with a Unit of Work, you might also choose to implement Undo / Rollback functionality. When using Entity Framework, the recommended approach to undo is to discard your context with the changes you are interested in undoing.
I already mentioned that our typed ObjectContext (NorthwindContext) supports the Unit of Work pattern for the most part. In order to make things a bit more explicit based on the contract I just defined, I can change my NorthwindContext class to implement the IUnitOfWork interface:
I have to make a small adjustment to our repository implementation after this change:
That’s it – we now have our IUnitOfWork friendly repository, and you can use the IUnitOfWork based context to even coordinate work across multiple repositories. Here’s an example of adding an order to the database that requires the work of multiple repositories for querying data, and ultimately saving rows back to the database:
What’s quite interesting to see is how little code we had to write in order to enable data access using Repository and Unit of Work patterns on top of Entity Framework. Entity Framework’s LINQ support and out of the box Unit of Work functionality makes it trivial to build repositories on top of Entity Framework.
Another thing to note is that a lot of what I’ve covered in this post has nothing to do with Entity Framework 4.0 specifically – all of these general principles will work with Entity Framework 3.5 as well. But being able to use Repository and Unit of Work like I have shown here on top of our POCO support is really telling of what’s possible in Entity Framework 4.0. I hope you find this useful.
Lastly, I’ll reiterate that there are many ways of approaching this topic, and there are many variations of solutions that will fit your needs when working with Entity Framework, Repository and Unit of Work. You might choose to implement a common IRepository interface. You might also choose to implement a Repository<TEntity> base class that gives you some common repository functionality across all your repositories.
Try some of the various approaches and see what works for you. What I’ve covered here is only a general guideline – but I hope it is enough to get you going. More importantly, I hope this shows you how it is possible to use the POCO support we have introduced in Entity Framework 4.0 to help you build a domain model that is Entity Framework friendly without needing you to compromise on the basic principles around persistence ignorance.
The project that includes some of the code I showed is attached to this post. Stay tuned as we’ll likely have more to say on this topic when we discuss unit testing, TDD and testability in one of our future posts.
Program Manager, Entity Framework