Question

I use Repository pattern (and linq2sql as data access) and have, for example, ProductsRep and CustomersRep.

In very simple scenario db has two tables – Produsts (ProductID, CustomerID, ProductName, Date) and Customer (CustomerID, FirstName, LastName).

Each repository provide methods to create, update, delete and get specific model object, and, may be, some filters.

But if I want to query all customers that buy specific product by product name, I have to get ProductID of this product using ProductsRep and then get all customers that buy product with this id using CustomersRep.

Am I right? This is actually two sql requests that l2s must generate, is it possible to do only one request?

And, in general, if we want to query data using multiple tables with relationships and repository pattern, how to do this by reduce amount of queries to minimum?

Thanks.

Answer 1

Think about the statement you made:

But if I want to query all customers
that buy specific product by product
name, I have to get ProductID of this
product using ProductsRep and then get
all customers that buy product with
this id using CustomersRep.

That smells trouble. Create a repository to facilitate behaviour in a isolated area of your domain, not for each business entity.

A common mistake with DDD/Repository is to think about data. DONT. Think about behaviour.

You have a Products entity which refers to a Customer. Which means a Product cannot exist without a customer. Right? Therefore a Customer is your aggregate root – it controls the behaviour of this portion of your domain model.

Your comment to the answer above confirms that – you are working across multiple repositories. You have two options – 1) unit of work, 2) fix up your repository design.

You only need 1 Repository – let’s call it CustomerProductRepository.

Then to do your above query:

var customersWhoBuyHats = customerProductRepository
                             .Products
                             .Include("Customer")
                             .Where(x => x.ProductName == "Hat")
                             .Select(x => x.Customer)
                             .ToList();

The result is a List<Customer>, and you did it with one database call.

HTH.