Entity Framework - DRY Queries - entity-framework

Question

I have a Entity Framework Domain model (using code first) with a context that includes Customers, and each Customer can have multiple Addresses with a 'calendar range' of dates on each address. I can write a query such as:
var query = from c in context.Customers
where c.CustomerId == 1
select c
Customer cust = query.Single();
The resulting customer is the one I selected via my Where clause. No problem. Now I also want to get their mailing address for a view, so I'd do:
var query = from c in context.Customers
where c.CustomerId == 1
select new
{
FirstName = c.FirstName,
Address = c.Addresses.Where(a => a.AddressStartDate > DateTime.Now &&
a.AddressEndDate < DateTime.Now)
}
var data = query.Single();
Address MailingAddress = data.Address;
Again, no problem, I get customer information and the current mailing address and the query executes in SQL.
Now I want to factor out the query that does the finding of the mailing address. I don't want to repeat it in every call that needs to get a mailing address. Ideally I'd like to add it to my domain object Customer so that the logic for finding mailing address is a part of my Customer object.
I want to place a method like this in my Customer object:
public partial class Customer
{
public Address MailingAddress
{
get
{
return (from a in this.Addresses.AsQueryable()
where a.AddressStartDate > DateTime.Now &&
a.AddressEndDate < DateTime.Now
select a).Single()
}
}
}
Now that I have the property in my Domain Model, I want to run a query:
var query = from c in context.Customers
where customerId == 1
select new
{
FirstName = c.FirstName,
Address = c.MailingAddress
}
Unfortunately this does not work with the error 'Only initializers, entity members, and entity navigation properties are supported.' I understand the error, and why I can't do it that way. I also know that my method would actually work if I retrieved the Customer first and then called the MailingAddress property after I had an instance of a Customer, but this doubles the database calls.
How can I stay DRY using Entity Framework? I need to centralize the code for the 'get mailing address' requirement while also making sure I execute the logic for 'get mailing address' in SQL and in one db operation?

Answer 1

The general way I've gone about solving this is to have a navigation property to the address so the class would look like:
public class Customer
{
public int Id { get; set; }
public int MailingAddressId { get; set; }
[ForeignKey("MailingAddressId")]
public virtual Address MailingAddress { get; set; }
public ICollection<Address> Addresses { get; set; }
}
Then you can go:
var customer = context.Customer.Include("MailingAddress").FirstOrDefault(c => c.Id = 1);