实体框架6投影，POCO实体返回类型

Question

Context: I have a solution with 3 projects

Project.Db: Contains my dbContext / Repositories
Project.Core: Contains my POCO classes (used by the dbContext).
Project.Web: My mvc project, which contains the viewModels and POCO-ViewModel mapping classes.

It seems impossible to have a typed projection return an EF entity type. In Project.Db I have the following method. I want to get two specific fields (Name and ID) of the regions and their subregions. I don't want the generated T-SQL to select the unwanted fields.

public List<Region> GetRegionsAndSubRegions(){
var regions = Context.Regions.Where(r => condition);
var lightRegions = regions.Select(z => new Region{
                                      RegionName = z.RegionName,
                                      RegionId = z.RegionId
                                  }).ToList();
}

This will get me the error

The entity or complex type 'Region' cannot be constructed in a LINQ to Entities query.

The way I found to get around is this:

var lightRegions = regions.ToList().Select(z => new Region{
                                      RegionName = z.RegionName,
                                      RegionId = z.RegionId
                                  }).ToList();

However, the "ToList()" will select all the fields, which I don't want.

I could project to a ViewModel class, but I don't want that dependency in Project.Db . The method should return a list of my POCO entity (List <Region >).

All my repository methods return Core (POCO) typed objects to the Web project.

Is there a way to do it without projecting to an anonymous type and map it back to Region type (which would be redundant)? Thanks

Answer 1

It's possible. I just did it with
<package id="EntityFramework" version="6.1.3" targetFramework="net461" />

A Vehicle(30+ fields in model & db) is projected to a StatisticsVehicle.

My Query:

var vehicles = 
    _context.Vehicles
        .Select(v => 
            new StatisticsVehicle()
            {
                PlateNumber = v.PlateNumber,
                StatisticsVehicleProcesses = 
                    v.VehicleProcesses.Select(vp => new StatisticsVehicleProcess()
                    {
                        EffectiveIssuanceDate = vp.EffectiveIssuanceDate,
                        PlannedIntakeEndDate = vp.PlannedIntakeEndDate
                    })
            }
        )
        .ToList();

Which results in a sql query only selecting the fields I need from that vehicle and vehicleprocess:

SELECT 
    [Project2].[Id] AS [Id], 
    [Project2].[C1] AS [C1], 
    [Project2].[PlateNumber] AS [PlateNumber], 
    [Project2].[C2] AS [C2], 
    [Project2].[EffectiveIssuanceDate] AS [EffectiveIssuanceDate], 
    [Project2].[PlannedIntakeEndDate] AS [PlannedIntakeEndDate]
    FROM ...

Answer 2

It is not possible to directly instantiate an EF Entity type in the projection. I think it is to prevent ambiguity like Gert said. The only way to do this is to project to anonymous type and then map to the entity type.

public List<Region> GetRegionsAndSubRegions(){
var regions = Context.Regions.Where(r => condition);
var anonRegions = regions.Select(r => new {
                                  RegionName = r.RegionName,
                                  RegionId = r.RegionId
                              }).ToList();
var lightRegions = anonRegions.Select(r => new Region{
                                  RegionName = r.RegionName,
                                  RegionId = r.RegionId
                              }).ToList();
}

Of course this is a pain in the neck and gets ugly real quick with a lot of table relationships.