如何在 Code First Entity Framework 中返回 DbGeography.Distance 计算值而不丢失强类型？

Question

目前我有一个通过 SqlGeography 列“可地理定位”的实体，我可以通过表达式使用它进行过滤和排序。 我已经能够获得点y距离x内的所有实体，并按最接近（或最远）点y 的实体排序。 但是，为了返回从实体到y的距离，我必须在应用程序中重新计算距离，因为我还没有确定如何实现从数据库到 IQueryable 中实体的距离计算结果。 这是一个映射实体，大量的应用程序逻辑围绕着返回的实体类型，因此将其投影到动态对象中对于此实现来说不是一个可行的选择（尽管我理解这是如何工作的）。 我还尝试使用从映射实体继承的未映射对象，但遇到同样的问题。 本质上，据我所知，如果我修改表示 IQueryable 的表达式树，但如何逃避我，我应该能够定义未映射属性的 getter 以在可查询扩展中分配计算值。 我以前以这种方式编写过表达式，但我认为我需要能够修改现有的选择，而不仅仅是链接一个新的 Expression.Call，这对我来说是未开发的领域。

以下应代码应正确说明问题：

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

换线怎么样

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字段在逻辑上不属于表格的一部分，因为它表示到动态指定点的距离。 因此，它不应该是您实体的一部分。

此时，如果您希望在 db 上计算它，您应该创建一个存储过程，或一个 TVF（或 sg 其他），返回您的实体随距离扩展。 通过这种方式，您可以将返回类型映射到实体。 顺便说一句，这对我来说是一个更清晰的设计。