Question

I work on an ASP.NET MVC product, using EF 4.3 with lazy loading enabled.

I’ve got a class Product, that can have zero or more Attachments:

public class Product
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public virtual long Id { get; set; }

    //... other properties

    public virtual List<Attachment> Pictures { get; set; }
}

public class Attachment
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public virtual long Id { get; set; }

    [Required]
    public virtual byte[] Data { get; set; }
}

My primary page (HTML) will serve product details, including a list of the attachments. For the latter, I only require the attachment’s id, and explicitly don’t want to load the Data property (which can be a lot of data).

How do I write a query that will only return the attachment’s id’s, and will not query the database for attachment’s data property?

The query below was not successful; error: Unable to create a constant value of type ‘XXX.Model.Attachment’. Only primitive types (‘such as Int32, String, and Guid’) are supported in this context.

var query = from a in db.Attachments
  from prod in db.Product
  from pic in prod.Pictures
  where prod.Id == prodId && a.Id == pic.Id
  select a.Id;
query.ToList();

And the following did load the attachment’s data field:

var product = db.Products.Find(prodId);
var pictureIds = product.Pictures.Select(x => x.Id).ToList();

Answer 1

Try this:

var query = from prod in db.Product
            from pic in prod.Pictures
            where prod.Id == prodId
            select pic.Id;
query.ToList();

or

var query = db.Product
              .Where(p => p.Id == prodId)
              .SelectMany(p => p.Pictures)
              .Select(a => a.Id)
              .ToList();