如何在 Code First Entity Framework 中返回 DbGeography.Distance 計算值而不丟失強類型？

Question

目前我有一個通過 SqlGeography 列“可地理定位”的實體，我可以通過表達式使用它進行過濾和排序。 我已經能夠獲得點y距離x內的所有實體，並按最接近（或最遠）點y 的實體排序。 但是，為了返回從實體到y的距離，我必須在應用程序中重新計算距離，因為我還沒有確定如何實現從數據庫到 IQueryable 中實體的距離計算結果。 這是一個映射實體，大量的應用程序邏輯圍繞着返回的實體類型，因此將其投影到動態對象中對於此實現來說不是一個可行的選擇（盡管我理解這是如何工作的）。 我還嘗試使用從映射實體繼承的未映射對象，但遇到同樣的問題。 本質上，據我所知，如果我修改表示 IQueryable 的表達式樹，但如何逃避我，我應該能夠定義未映射屬性的 getter 以在可查詢擴展中分配計算值。 我以前以這種方式編寫過表達式，但我認為我需要能夠修改現有的選擇，而不僅僅是鏈接一個新的 Expression.Call，這對我來說是未開發的領域。

以下應代碼應正確說明問題：

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity;
using System.Data.Entity.ModelConfiguration;
using System.Data.Entity.Spatial; // from Microsoft.SqlServer.Types (Spatial) NuGet package
using System.Linq;

public class LocatableFoo
{
    [Key]
    public int Id { get; set; }

    public DbGeography Geolocation { get; set; }

    [NotMapped]
    public double? Distance { get; set; }
}

public class PseudoLocatableFoo : LocatableFoo
{
}

public class LocatableFooConfiguration : EntityTypeConfiguration<LocatableFoo>
{
    public LocatableFooConfiguration()
    {
        this.Property(foo => foo.Id).HasColumnName("id");
        this.Property(foo => foo.Geolocation).HasColumnName("geolocation");
    }
}

public class ProblemContext : DbContext
{
    public DbSet<LocatableFoo> LocatableFoos { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add(new LocatableFooConfiguration());

        base.OnModelCreating(modelBuilder);
    }
}

public class Controller
{
    public Controller(ProblemContext context) // dependency injection
    {
        this.Context = context;
    }

    private ProblemContext Context { get; set; }

    /* PROBLEM IN THIS METHOD:
     * Do not materialize results (ie ToList) and then calculate distance as is done currently <- double calculation of distance in DB and App I am trying to solve
     * Must occur prior to materialization
     * Must be assignable to "query" that is to type IQueryable<LocatableFoo>
     */
    public IEnumerable<LocatableFoo> GetFoos(decimal latitude, decimal longitude, double distanceLimit)
    {
        var point = DbGeography.FromText(string.Format("Point({0} {1})", longitude, latitude), 4326); // NOTE! This expects long, lat rather than lat, long.
        var query = this.Context.LocatableFoos.AsQueryable();

        // apply filtering and sorting as proof that EF can turn this into SQL
        query = query.Where(foo => foo.Geolocation.Distance(point) < distanceLimit);
        query = query.OrderBy(foo => foo.Geolocation.Distance(point));

        //// this isn't allowed because EF doesn't allow projecting to mapped entity
        //query = query.Select( foo => new LocatableFoo { Id = foo.Id, Geolocation = foo.Geolocation, Distance = foo.Geolocation.Distance(point) });

        //// this isn't allowed because EF doesn't allow projecting to mapped entity and PseudoLocatableFoo is considered mapped since it inherits from LocatableFoo
        //query = query.Select( foo => new PseudoLocatableFoo { Id = foo.Id, Geolocation = foo.Geolocation, Distance = foo.Geolocation.Distance(point) });

        //// this isn't allowed because we must be able to continue to assign to query, type must remain IQueryable<LocatableFoo>
        //query = query.Select( foo => new { Id = foo.Id, Geolocation = foo.Geolocation, Distance = foo.Geolocation.Distance(point) });

        // this is what I though might work
        query = query.SelectWithDistance(point);

        this.Bar(query);
        var results = query.ToList(); // run generated SQL
        foreach (var result in results) //problematic duplicated calculation
        {
            result.Distance = result.Geolocation.Distance(point);
        }

        return results;
    }

    // fake method representing lots of app logic that relies on knowing the type of IQueryable<T>
    private IQueryable<T> Bar<T>(IQueryable<T> foos)
    {
        if (typeof(T) == typeof(LocatableFoo))
        {
            return foos;
        }

        throw new ArgumentOutOfRangeException("foos");
    }
}

public static class QueryableExtensions
{
    public static IQueryable<T> SelectWithDistance<T>(this IQueryable<T> queryable, DbGeography pointToCalculateDistanceFrom)
    {
        /* WHAT DO?
         * I'm pretty sure I could do some fanciness with Expression.Assign but I'm not sure
         * What to get the entity with "distance" set
         */
        return queryable;
    }
}

Answer 1

換線怎么樣

var results = query.ToList();

和

var results = query
                .Select(x => new {Item = x, Distance = x.Geolocation.Distance(point)}
                .AsEnumerable() // now you just switch to app execution
                .Select(x => 
                {
                    x.Item.Distance = x.Distance; // you don't need to calculate, this should be cheap
                    return x.Item;
                })
                .ToList();

Answer 2

Distance字段在邏輯上不屬於表格的一部分，因為它表示到動態指定點的距離。 因此，它不應該是您實體的一部分。

此時，如果您希望在 db 上計算它，您應該創建一個存儲過程，或一個 TVF（或 sg 其他），返回您的實體隨距離擴展。 通過這種方式，您可以將返回類型映射到實體。 順便說一句，這對我來說是一個更清晰的設計。