c# - Linq Extension Methods with Entity Framework 初学者错误

Question

我正在使用 Entity Framework 编写我的第一个应用程序，但迷路了。我遵循了类似线程的建议，但没有成功。

我想在我的服务方法中执行以下操作(只获取所有数据):

 using (WeatherStationDbContext entityContext = new WeatherStationDbContext())
            {
                var weatherData = entityContext.Weather
                                                .Include(x => x.Sensor)
                                                .Include(x => x.Position.Select(p => p.Location))
                                                .ToList();

(...)

并且在 .Include(x => x.Position.Select(p => p.Location)) 中出现错误:“位置”不包含“选择”的定义，也没有扩展方法“选择”接受...

我遵循了 for e.g. 的代码https://msdn.microsoft.com/en-us/library/jj574232(v=vs.113).aspx

// Load all blogs, all related posts, and all related comments 
var blogs1 = context.Blogs 
                   .Include(b => b.Posts.Select(p => p.Comments)) 
                   .ToList(); 

我的类(class):

public class Location
{
    public int Id { get; set; }

    public string Description { get; set; }

    public int Width { get; set; }

    public int Length { get; set; }
}

public class Position
{
    public int Id { get; set; }

    public int SensorId { get; set; }

    public virtual Sensor Sensor { get; set; }

    public int LocationId { get; set; }

    public virtual Location Location { get; set; }

    public string Description { get; set; }

    public float CoordinateX { get; set; }

    public float CoordinateY { get; set; }

}

public class Sensor
{
    public int Id { get; set; }

    public string Description { get; set; }
}

public class Weather
{
    public int Id { get; set; }

    public int SensorId { get; set; }

    public virtual Sensor Sensor { get; set; }

    public int PositionId { get; set; }

    public virtual Position Position { get; set; }

    public float Temperature { get; set; }

    public float Humidity { get; set; }

    public DateTime Date { get; set; }
}

在 DbContext 中:

public DbSet<Sensor> Sensor { get; set; }
public DbSet<Location> Location { get; set; }
public DbSet<Position> Position { get; set; }
public DbSet<Weather> Weather { get; set; }

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);
    modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();

    modelBuilder.Entity<Weather>()
                .HasRequired(c => c.Sensor)
                .WithMany()
                .WillCascadeOnDelete(false);

    modelBuilder.Entity<Weather>()
                .HasRequired(c => c.Position)
                .WithMany()
                .WillCascadeOnDelete(false);

}

Answer 1

只需将您的 Linq 更改为以下内容:

var weatherData = entityContext.Weather
                                .Include(x => x.Sensor)
                                .Include(x => x.Position.Location)
                                .ToList();

您遇到问题的原因是 Position 不是集合。如 DbExtensions.Include Method 的 MSDN 文档中所述:

Remarks

The path expression must be composed of simple property access expressions together with calls to Select in order to compose additional includes after including a collection property. Examples of possible include paths are:

• To include a reference and then a reference one level down: query.Include(e => e.Level1Reference.Level2Reference).

• To include a collection and then a reference one level down: query.Include(e => e.Level1Collection.Select(l1 => l1.Level2Reference)).