如何仅从Entity Framework中的对象图中选择特定属性？ [英] How to only select specific properties from an object graph in Entity Framework?

问题描述

我有一个简单的数据模型

I have a simple data model

car
 - make
 - model
 - year
 - colour
 - engine
     - model
     - no. cylinders
     - size
     - etc
 - fuel tank
     - model
     - capacity
     - fuel type
     - etc
 - etc

所以我有汽车，发动机和油箱实体。每个都有许多属性。

So I have 'car', 'engine' and 'fuel tank' entities. Each of which have many properties.

我想要列出所有100辆汽车，但只想显示以下选定的属性： car.make，car.model，car.year，car.engine，car.size，car.fueltype 。

I want a list of all the 100s of cars but only want to show the following selected properties: car.make, car.model, car.year, car.engine, car.size, car.fueltype.

我当然可以使用 .include 来带回对象图中的子实体，但这是一个很大的成功，因为有很多属性。

I can certainly use .include to bring back sub-entities in the object graph but this is a big hit as there are many properties.

我的问题是是否有一种整洁的方法来做到这一点。还是实际上使用实体框架（最好是EF7 / Core）的任何方式？

My question is whether there is a neat way to do this. Or any way in fact using Entity Framework (ideally EF7/Core)?

[我确实指的是 https://colinmackay.scot/2011/07/31/getting-just-the-columns-you-want-from-entity-framework/ 它使用select进入一个匿名类，但看不到它如何在多个include中起作用]

[ I did refer to https://colinmackay.scot/2011/07/31/getting-just-the-columns-you-want-from-entity-framework/ which uses the select into an anonymous class, but could not see how this could work within multiple includes ]

谢谢。

推荐答案

如果您想拉回完整实体，则只需要使用 Include -您不要需要这些来进行投影。您可以匿名进行投影，也可以使用定义的模型类进行投影。以下代码应该使您入门：

You only need to use Include if you want to pull the full entities back - you don't need these to do a projection. You can do a projection either anonymously, or using a defined model class. The following code should get you started:

// Define model...
public class CarModel
{
    public string Make { get; set; }
    public string Model { get; set; }
    public int EngineCC { get; set; }
}

// Project to list of models
var cars = context.Cars.Select(c => new CarModel
{
    Make = c.Make,
    Model = c.Model,
    EngineCC = c.Engine.CC
}).ToList();

您可以使用 AutoMapper 。使用AutoMapper，它变为：

You can make this much simpler by using a mapping library such as AutoMapper. Using AutoMapper, this becomes:

// (at start of project)
Mapper.Initialize(c => {
    c.CreateMap<Car, CarModel>();
});

// Projection...
var cars = context.Cars.ProjectTo<CarModel>().ToList();

在此示例中，EngineCC是从Engine.CC自动映射的，但是您可以手动指定任何映射不只是自动工作。 AutoMapper将创建一个Linq投影，仅恢复您需要的属性。

In this example, EngineCC was automatically mapped from Engine.CC, but you can manually specify any mappings which don't just work automatically. AutoMapper will create a Linq projection, only bringing back the properties you need.

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