Repository Pattern, POCO, and Business Entities

Question

I know there are a lot of threads here already on the repository pattern but somehow I feel my question is a little different. May be because yesterday was the first time I heard of the word POCO.

My question is this--usually, I have add and save methods in my business entities. Say I am writing a Q/A site, and I have the following entities: questions, answers, and comments. If I wanted to use the repository pattern, I basically need to keep only the properties in my business entities (example, Question), and move my operations to the repository class (example, QuestionRepository), right? If this is true, does POCO mean a business entity with just the properties?

I'm using Entity Framework 4.0, which created my entities in the edmx code behind. If I wanted to use the repository pattern with this, there is no need to write my own business entities (Question, Answer, etc) since they are already generated by EF, right? All I'd need is the Repository to do CRUD? And I'll be having three repositories for this example, one for each entity?

Answer 1

Your POCO objects would still have methods on them for operations, but those operations would pertain to the business concerns of the entity not the persistence concerns(CRUD). If there are no business operations on your entites, then yes they will be just properties.

You wouldn't need to write your pocos from scratch if you are generating them but you may want to extend them with partial classes for business operations and non-persistent or calculated properties.

You could have a single repository class or a repository for each entity.

Answer 2

First an observation about MVC pattern

I must say that Visual Studio has a tiny flaw in Asp.net MVC project template. And that is the Model folder. People not knowing MVC pattern would automatically relate this to data model and not MVC application model. This is fine for simple applications but not for anything else.

Let's continue to my answer

When I write business level applications I separate my solution into 4 projects (at least):

• presentation tier - Asp.net MVC application but I remove Model folder and have all my views as strong type views to avoid magic strings as much as possible
• service tier - business logic processes
• data tier - data model ie. EF4 and repositories that access this model
• objects tier - this project actually has POCOs that are used for inter-project communication

My request process usually looks very clean and works this way:

1. When a request is made my Asp.net MVC controller action validates data, does whatever is necessary for the presentation tier before calling into services.
2. Service is called that does whatever business process logic requires and normally calls repository to do something with data.
3. Repository manipulates data in the data model and then creates POCOs from results that will be returned to service layer.
4. Service layer receives POCOs does additional logic if needed and returns them back to presentation.
5. Presentation (controller) decides which view to display and provides model for that particular view and returns it. Of course instead of a view it can be any other result as well.

Advantage of using separate MVC model classes in Objects project (you couldn't put them in the Model folder because of circular project reference) is that I can have presentation optimised classes. Or better said: I have business process centric interface instead of data centric.

Let's emplain this with an example: Take for instance user registration view. It can't be strong typed to data model's User entity. Why? Because it has two inputs for password. So I can have a POCO application model class called UserRegistration even though there's nothing similar in data model. Its validation works completely different compared to data model's User entity. If I'd have my user registration done without strong type I'd have to make my controller action with all parameters of every single field. These wouldn't be automatically validated which would mean that I can have a bigger surface for bugs. Someone might hurry into writing code but forgets certain aspects of validation.

Strong type views returning strong types back at server are the safest way of getting rid of all kinds of obscure bugs that are usually discovered by users especially if you don't do any methodical testing on your project (which is somewhere between 75-90% chance).