EF Core 2.2 Entity Nested Projection

Question

EF Core 2.2 Entity Nested Projection

.net-core c# ef-core-2.2 entity-framework-core projection

Question

I have three related entities A, B and C where A is the parent of B and B is the parent of C.

    public class A
    {
        public int Id { get; set; }
        public B B { get; set; }
    }

    public class B
    {
        public int Id { get; set; }
        public C C { get; set; }
    }

    public class C
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

The entities above have their own DTO models which contain FromEntity method and Projection.

    public class ADto
    {
        public int Id { get; set; }
        public BDto BDto { get; set; }

        public static Expression<Func<A, ADto>> Projection
        {
            get
            {
                return x => new ADto
                {
                    Id = x.Id,
                    BDto = BDto.FromEntity(x.B)
                };
            }
        }
    }

    public class BDto
    {
        public int Id { get; set; }
        public CDto CDto { get; set; }

        public static Expression<Func<B, BDto>> Projection
        {
            get
            {
                return x => new BDto
                {
                    Id = x.Id,
                    CDto = CDto.FromEntity(x.C)
                };
            }
        }

        public static BDto FromEntity(B entity)
        {
            return Projection.Compile().Invoke(entity);
        }
    }

    public class CDto
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public static Expression<Func<C, CDto>> Projection
        {
            get
            {
                return x => new CDto
                {
                    Id = x.Id,
                    Name = x.Name
                };
            }
        }

        public static CDto FromEntity(C entity)
        {
            return Projection.Compile().Invoke(entity);
        }
    }

Then I use projection like this:

_context.A.Where(x = x.Id == id).Select(ADto.Projection).FirstOrDefault();

All my current attempts ended up with the exception in CDto in FromEntity method, because the entity is null and therefore the projection can not be invoked.

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Is there any way to chain more nested projections?

I am aware that I can use this:

    public class ADto
    {
        public int Id { get; set; }
        public BDto BDto { get; set; }

        public static Expression<Func<A, ADto>> Projection
        {
            get
            {
                return x => new ADto
                {
                    Id = x.Id,
                    BDto = new BDto()
                    {
                        Id = x.B.Id,
                        CDto = new CDto()
                        {
                            Id = x.B.C.Id,
                            Name = x.B.C.Name
                        }
                    }
                };
            }
        }
    }

But I would like to manage projection in one place, because real classes are too complicated.

Martin VaÅ™eÄka

View more on Stack Overflow

umutesen · Accepted Answer · 4/17/2019 1:27:06 PM

I have been using Automapper for quite some time in .NET Core projects due to ease of use and built-in dependency injection. It can handle nested object mappings.

Install from PM:

Install-Package AutoMapper
Install-Package AutoMapper.Extensions.Microsoft.DependencyInjection

Register in the Startup.cs, ConfigureServices method:

services.AddAutoMapper(typeof(Startup));

Create a class to keep your mappings, e.g. MappingProfile.cs using Profile from automapper, you can define mappings.

public class MappingProfile : Profile
{
    public MappingProfile()
    {
        CreateMap<A, ADto>();
        CreateMap<B, BDto>();
        CreateMap<C, CDto>();
    }
}

The above mapping tells automapper that the entity can be mapped to DTO.

In your controller, you can inject an IMapper

private readonly IMapper _mapper;

public MyController(IMapper mapper)
{
    _mapper = mapper;
}

and map values like below:

var dto = _mapper.Map<A>(a); // Map object to dto, which will map nested objects.

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