AutoMapper，将 collections 的嵌套集合映射到列表

Question

I have just started using AutoMapper on an asp net core project and I'm trying to map an object that has a collection of an object that also has a collection of an object to an entity.

The source

public class MyClass
    {
        public List<MyCollection> MyCollections { get; set; }
    }

    public class MyCollection
    {
        public int CollectionId { get; set; }
        public List<Description> Descriptions { get; set; }
    }

    public class Description
    {
        public int DescriptionId { get; set; }
        public string Text { get; set; }
    }

The destination

 public class DescriptionToCollection
    {
        public int DescriptionId { get; set; }
        public int CollectionId { get; set; }
    }

I've played around with ConvertUsing thinking something like this, but I can't make it work.

CreateMap<MyClass, List<DescriptionToCollection>>()
             .ConvertUsing(source => source.MyCollections.Select(x =>x.Description.Select(y=> new DescriptionToCollection{ DescriptionId=y.DescriptionId,CollectionId=x.CollectionId}).ToList()
             ));

Searching AutoMappers docs and the internet, I couldn't find anything similar to my problem. Any help is highly appreciated.

Answer 1

Try to implement ITypeConverter, follow the example code:

Your Classes

    public class Class1
    {
        public List<Class2> class2 { get; set; }
    }

    public class Class2
    {
        public int CollectionId { get; set; }
        public List<Class3> class3 { get; set; }
    }

    public class Class3
    {
        public int DescriptionId { get; set; }
        public string Text { get; set; }
    }


    public class ClassDto
    {
        public int DescriptionId { get; set; }
        public int CollectionId { get; set; }
    }

The custom map

    public class ClassCustomMap : ITypeConverter<Class1, List<ClassDto>>
    {

        public List<ClassDto> Convert(Class1 source, List<ClassDto> destination, ResolutionContext context)
        {
            var classDtoList = new List<ClassDto>();
            foreach (var item in source.class2)
            {
                var collectionID = item.CollectionId;
                foreach (var obj in item.class3)
                {
                    var classDto = new ClassDto();
                    classDto.CollectionId = collectionID;
                    classDto.DescriptionId = obj.DescriptionId;

                    classDtoList.Add(classDto);
                }
            }

            return classDtoList;
        }
    }

The mapping declaration

    CreateMap<Class1, List<ClassDto>>().ConvertUsing<ClassCustomMap>();

How to use it

    var class2 = new Class2();
    class2.CollectionId = 2;

    var class3 = new Class3();
    class3.DescriptionId = 1;
    class3.Text = "test";

    class2.class3 = new System.Collections.Generic.List<Class3>() { class3 };

    var class1 = new Class1();
    class1.class2 = new System.Collections.Generic.List<Class2>() { class2 };

    var result = mapper.Map<List<ClassDto>>(class1);

For clarity and to simplify I have used explicit cycles, if you want you can optimize the conversion function using LinQ and Lambda

Answer 2

Besides a typo in your example code, you almost had it. Because you aren't mapping 1:1 at the top level, you need to flatten somewhere, and you do that using SelectMany , moving the ToList call appropriately.

CreateMap<MyClass, List<DescriptionToCollection>>()
    .ConvertUsing(source => source.MyCollections.SelectMany(x => // SelectMany to flatten
                        x.Descriptions.Select(y => 
                            new DescriptionToCollection
                            {
                                DescriptionId = y.DescriptionId,
                                CollectionId = x.CollectionId
                            }
                        ) // ToList used to be here
                    ).ToList()
                );

Answer 3

You are missing the purpose of auto-mapper.

It should be used for transforming an input object of one type into an output object of a different type.

And you wanted to create a map from MyClass type to List - this should be treated as projection.

You can achieve that using LINQ (for example as a extension method on MyClass):

    public static class MyClassExtension
    {
        public static List<DescriptionToCollection> ToDescriptionToCollection(this MyClass value)
        {
            return value.MyCollections.SelectMany(mc => mc.Descriptions.Select(d => new DescriptionToCollection()
            {
                CollectionId = mc.CollectionId,
                DescriptionId = d.DescriptionId
            })).ToList();
        }
    }