Using Entity Framework with additional property

Question

I have 2 tables, one named Products and another named Samples, with a foreign key to a product. A sample simply consist of a decimal value and a timestamp. A product have a name and that's it. But a I want a price (decimal) to be associated with a product. The price should be the latest sample in Samples.

I am interested in using Entity Framework, but how can I achieve a Product Entity with a price value, when it is not directly in the table and some "calculations" need to be performed beforehand?

You can create a partial class for Samples and add any properties that you want. — Tyler
– Tyler, Commented Jul 2, 2014 at 20:56

Pharylon · Accepted Answer · 2014-07-02 21:05:50Z

1

There's an attribute called NotMapped that will let you create a property that's, well, not mapped to the database. Doing it code-first, something like this should work:

public class Product
{
    public int Id { get; set; }
    public virtual IEnumerable<Sample> Samples { get; set; }
    [NotMapped]
    public decimal Price
    {
        get { return Samples.OrderByDescending(x => x.TimeStamp).First().Price; }
    }
}

public class Sample
{
    public int Id {get; set;}
    public decimal Price { get; set; }
    [Timestamp]
    public byte[] TimeStamp {get; set;}
}

If it's not code-first, the classes EF builds for you are partial classes, so you can easily extend it similarly.

answered Jul 2, 2014 at 21:05

Pharylon

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6 Comments

Neil Smith Over a year ago

Alternatively, if you're doing configuration with the Fluent API instead of attributes, you can use the Ignore method.

Peter Over a year ago

But will the queries not be done in memory and not in sql database then? I need great performance because I have a huge set of data, so I cannot neglect performance

Neil Smith Over a year ago

@Peter Since Samples is marked virtual, it's lazy loaded. So anytime you have a Product you will already have its related Samples. The Price property just returns the most recent Sample from that collection, not the DB.

Peter Over a year ago

Oh, nice. Do I need to do something different when I do db first? When I try iterating over my products with the [notmapped] prop its way slower than without it. Just makes me nervous

Neil Smith Over a year ago

@Peter Is it ICollection or ICollection<T>? I would expect it to be the second which does provide OrderBy. If its the first, you can cast it to the generic type by doing ICollection.Cast<T>() which you can then use OrderBy with. Check here for an answer about casting.

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