How to only select specific properties from an object graph in Entity Framework?

问题

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

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

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.

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.

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

[ 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 ]

Thanks you.

回答1:

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();

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();

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.

来源：https://stackoverflow.com/questions/36573224/how-to-only-select-specific-properties-from-an-object-graph-in-entity-framework

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entity-framework-core