Question

I am performing distance calculations between addresses. Specifically the distance between a User and a particular location. I can build the LINQ queries that cause the database to calculate the distance and order the results by that distance. However, I can’t find a way to RETURN that distance in the Entities. A simplified version of the query is below:

    public IQueryable<Place> GetPlacesWithinRadiusOfUser(double userLongitude, double userLatitude, int miles )
    {

        return (from Place in this.ObjectContext.Places
                where RadiusOfEarth * Math.Acos((double)
                    (Math.Sin((double)Place.Addresses.First().Latitude / DegreesToRadians) * Math.Sin(userLatitude / DegreesToRadians))
                    +
                    (Math.Cos((double)Place.Addresses.First().Latitude / DegreesToRadians) * Math.Cos(userLatitude / DegreesToRadians)
                    * Math.Cos((userLongitude / DegreesToRadians) - ((double)Place.Addresses.First().Longitude / DegreesToRadians))
                    )
                    )
                    <= miles
                select Place);


    }

Essentially, the math from above I need to return as a Distance property on the Address Entity (which is child of Place). I have created the following partial class and the field does appear in the silverlight client.

public partial class Address
{
    //The Distance field is used to return a distance if a reference point is provided in a query
    [DataMember]
    public Nullable<decimal> Distance { get; set; }

}

So, the Distance property and calculation is dependent on the parameters of the GetPlacesWithinRadiusOfUser method.

Is there a way to get the parameters from the query to the Address Entity so that I can perform the same calculation and have it available in the Entity upon arriving at the client?

I can certainly create a client side routine that will do this calculation but this will get messy as I will have to continually inject that in every UI that needs it. I would much prefer to return it in the Entity so that I can just bind to it and display it (essentially I want a calculated field returned from SQL). If it were striaght SQL, this would be trivial but I am at a loss as to how to do this with RIA Services and the Entity Framework.

And thoughts?

Answer 1

With Entity Framework, I was never able to find a way to actually do this (the way I wanted to). However, I found a pretty good solution. Steps are as follows:

1. Created the query by issuing the distance calculation on the ObjectContext….then filtered a Place query by the returned values. All of that is done in one query call from the client. It doesn’t return the distance…but I handled that a different way.

2. I then created extension methods on all of the relavent objects like so:

   public static class AddressExtensions
   {
   
   
   public static double CalculateDistanceTo(this Address address1, double longitude, double latitude)
   {
       if (address1 == null)
       {
           return 0;
       }
   
       return Global.CalculateDistance((double?)address1.Longitude, (double?)address1.Latitude, longitude, latitude);
   
   }
   
   public static double CalculateDistanceTo(this Address address1, Address address)
   {
       if (address1 == null || address == null)
       {
           return 0;
       }
   
       return Global.CalculateDistance((double?)address1.Longitude, (double?)address1.Latitude, (double?)address.Longitude, (double?)address.Latitude);
   
   }
   
   public static double CalculateDistanceTo(this Address address1, User user)
   {
       if (address1 == null)
       {
           return 0;
       }
   
       return address1.CalculateDistanceTo(user.Addresses.FirstOrDefault());
   
   }
   
   public static double CalculateDistanceTo(this Address address1, Place place)
   {
       if (address1 == null)
       {
           return 0;
       }
   
       return address1.CalculateDistanceTo(place.Addresses.FirstOrDefault());
   
   }
   }
   

3. Then, anywhere I needed to get the distance, I could do something like User.DistanceTo(myAddress)

This actually worked out a little better because I could then reuse the same extension methods no matter what the situation.