ASP.NET Core with EF Core - DTO Collection mapping

Question

I am trying to use (POST/PUT) a DTO object with a collection of child objects from JavaScript to an ASP.NET Core (Web API) with an EF Core context as my data source.

The main DTO class is something like this (simplified of course):

public class CustomerDto {
    public int Id { get;set }
    ...
    public IList<PersonDto> SomePersons { get; set; }
    ...
}

What I don't really know is how to map this to the Customer entity class in a way that does not include a lot of code just for finding out which Persons had been added/updated/removed etc.

I have played around a bit with AutoMapper but it does not really seem to play nice with EF Core in this scenario (complex object structure) and collections.

After googling for some advice around this I haven't found any good resources around what a good approach would be. My questions is basically: should I redesign the JS-client to not use "complex" DTOs or is this something that "should" be handled by a mapping layer between my DTOs and Entity model or are there any other good solution that I am not aware of?

I have been able to solve it with both AutoMapper and and by manually mapping between the objects but none of the solutions feels right and quickly become pretty complex with much boilerplate code.

EDIT:

The following article describes what I am referring to regarding AutoMapper and EF Core. Its not complicated code but I just want to know if it's the "best" way to manage this.

(Code from the article is edited to fit the code example above)

http://cpratt.co/using-automapper-mapping-instances/

var updatedPersons = new List<Person>();
foreach (var personDto in customerDto.SomePersons)
{
    var existingPerson = customer.SomePersons.SingleOrDefault(m => m.Id == pet.Id);
    // No existing person with this id, so add a new one
    if (existingPerson == null)
    {
        updatedPersons.Add(AutoMapper.Mapper.Map<Person>(personDto));
    }
    // Existing person found, so map to existing instance
    else
    {
        AutoMapper.Mapper.Map(personDto, existingPerson);
        updatedPersons.Add(existingPerson);
    }
}
// Set SomePersons to updated list (any removed items drop out naturally)
customer.SomePersons = updatedPersons;

Code above written as a generic extension method.

public static void MapCollection<TSourceType, TTargetType>(this IMapper mapper, Func<ICollection<TSourceType>> getSourceCollection, Func<TSourceType, TTargetType> getFromTargetCollection, Action<List<TTargetType>> setTargetCollection)
    {
        var updatedTargetObjects = new List<TTargetType>();
        foreach (var sourceObject in getSourceCollection())
        {
            TTargetType existingTargetObject = getFromTargetCollection(sourceObject);
            updatedTargetObjects.Add(existingTargetObject == null
                ? mapper.Map<TTargetType>(sourceObject)
                : mapper.Map(sourceObject, existingTargetObject));
        }
        setTargetCollection(updatedTargetObjects);
    }

.....

        _mapper.MapCollection(
            () => customerDto.SomePersons,
            dto => customer.SomePersons.SingleOrDefault(e => e.Id == dto.Id),
            targetCollection => customer.SomePersons = targetCollection as IList<Person>);

Edit:

One thing I really want is to delcare the AutoMapper configuration in one place (Profile) not have to use the MapCollection() extension every time I use the mapper (or any other solution that requires complicating the mapping code).

So I created an extension method like this

public static class AutoMapperExtensions
{
    public static ICollection<TTargetType> ResolveCollection<TSourceType, TTargetType>(this IMapper mapper,
        ICollection<TSourceType> sourceCollection,
        ICollection<TTargetType> targetCollection,
        Func<ICollection<TTargetType>, TSourceType, TTargetType> getMappingTargetFromTargetCollectionOrNull)
    {
        var existing = targetCollection.ToList();
        targetCollection.Clear();
        return ResolveCollection(mapper, sourceCollection, s => getMappingTargetFromTargetCollectionOrNull(existing, s), t => t);
    }

    private static ICollection<TTargetType> ResolveCollection<TSourceType, TTargetType>(
        IMapper mapper,
        ICollection<TSourceType> sourceCollection,
        Func<TSourceType, TTargetType> getMappingTargetFromTargetCollectionOrNull,
        Func<IList<TTargetType>, ICollection<TTargetType>> updateTargetCollection)
    {
        var updatedTargetObjects = new List<TTargetType>();
        foreach (var sourceObject in sourceCollection ?? Enumerable.Empty<TSourceType>())
        {
            TTargetType existingTargetObject = getMappingTargetFromTargetCollectionOrNull(sourceObject);
            updatedTargetObjects.Add(existingTargetObject == null
                ? mapper.Map<TTargetType>(sourceObject)
                : mapper.Map(sourceObject, existingTargetObject));
        }
        return updateTargetCollection(updatedTargetObjects);
    }
}

Then when I create the mappings I us it like this:

    CreateMap<CustomerDto, Customer>()
        .ForMember(m => m.SomePersons, o =>
        {
            o.ResolveUsing((source, target, member, ctx) =>
            {
                return ctx.Mapper.ResolveCollection(
                    source.SomePersons,
                    target.SomePersons,
                    (targetCollection, sourceObject) => targetCollection.SingleOrDefault(t => t.Id == sourceObject.Id));
            });
        });

Which allow me to use it like this when mapping:

_mapper.Map(customerDto, customer);

And the resolver takes care of the mapping.

Answer 1

AutoMapper is the best solution.

You can do it very easily like this :

    Mapper.CreateMap<Customer, CustomerDto>();
    Mapper.CreateMap<CustomerDto, Customer>();

    Mapper.CreateMap<Person, PersonDto>();
    Mapper.CreateMap<PersonDto, Person>();

Note : Because AutoMapper will automatically map the List<Person> to List<PersonDto>.since they have same name, and there is already a mapping from Person to PersonDto.

If you need to know how to inject it to ASP.net core,you have to see this article : Integrating AutoMapper with ASP.NET Core DI

Auto mapping between DTOs and entities

Mapping using attributes and extension methods

Answer 2

I was struggling with the very same issue for quite some time. After digging through many articles I've came up with my own implementation which I'm sharing with you.

First of all I've created a custom IMemberValueResolver.

using System;
using System.Collections.Generic;
using System.Linq;

namespace AutoMapper
{
    public class CollectionValueResolver<TDto, TItemDto, TModel, TItemModel> : IMemberValueResolver<TDto, TModel, IEnumerable<TItemDto>, IEnumerable<TItemModel>>
        where TDto : class
        where TModel : class
    {
        private readonly Func<TItemDto, TItemModel, bool> _keyMatch;
        private readonly Func<TItemDto, bool> _saveOnlyIf;

        public CollectionValueResolver(Func<TItemDto, TItemModel, bool> keyMatch, Func<TItemDto, bool> saveOnlyIf = null)
        {
            _keyMatch = keyMatch;
            _saveOnlyIf = saveOnlyIf;
        }

        public IEnumerable<TItemModel> Resolve(TDto sourceDto, TModel destinationModel, IEnumerable<TItemDto> sourceDtos, IEnumerable<TItemModel> destinationModels, ResolutionContext context)
        {
            var mapper = context.Mapper;

            var models = new List<TItemModel>();
            foreach (var dto in sourceDtos)
            {
                if (_saveOnlyIf == null || _saveOnlyIf(dto))
                {
                    var existingModel = destinationModels.SingleOrDefault(model => _keyMatch(dto, model));
                    if (EqualityComparer<TItemModel>.Default.Equals(existingModel, default(TItemModel)))
                    {
                        models.Add(mapper.Map<TItemModel>(dto));
                    }
                    else
                    {
                        mapper.Map(dto, existingModel);
                        models.Add(existingModel);
                    }
                }
            }

            return models;
        }
    }
}

Then I configure AutoMapper and add my specific mapping:

cfg.CreateMap<TDto, TModel>()
    .ForMember(dst => dst.DestinationCollection, opts =>
        opts.ResolveUsing(new CollectionValueResolver<TDto, TItemDto, TModel, TItemModel>((src, dst) => src.Id == dst.SomeOtherId, src => !string.IsNullOrEmpty(src.ThisValueShouldntBeEmpty)), src => src.SourceCollection));

This implementation allows me to fully customize my object matching logic due to keyMatch function that is passed in constructor. You can also pass an additional saveOnlyIf function if you for some reason need to verify passed objects if they are suitable for mapping (in my case there were some objects that shouldn't be mapped and added to collection if they didn't pass an extra validation).

Then e.g. in your controller if you want to update your disconnected graph you should do the following:

var model = await Service.GetAsync(dto.Id); // obtain existing object from db
Mapper.Map(dto, model);
await Service.UpdateAsync(model);

This works for me. It's up to you if this implementation suits you better than what author of this question proposed in his edited post:)

Answer 3

First I would recommend using JsonPatchDocument for your update:

    [HttpPatch("{id}")]
    public IActionResult Patch(int id, [FromBody] JsonPatchDocument<CustomerDTO> patchDocument)
    {
        var customer = context.EntityWithRelationships.SingleOrDefault(e => e.Id == id);
        var dto = mapper.Map<CustomerDTO>(customer);
        patchDocument.ApplyTo(dto);
        var updated = mapper.Map(dto, customer);
        context.Entry(entity).CurrentValues.SetValues(updated);
        context.SaveChanges();
        return NoContent();
    }

And secound you should take advantage of AutoMapper.Collections.EFCore. This is how I configured AutoMapper in Startup.cs with an extension method, so that I´m able to call services.AddAutoMapper() without the whole configuration-code:

    public static IServiceCollection AddAutoMapper(this IServiceCollection services)
    {
        var config = new MapperConfiguration(cfg =>
        {
            cfg.AddCollectionMappers();
            cfg.UseEntityFrameworkCoreModel<MyContext>(services);
            cfg.AddProfile(new YourProfile()); // <- you can do this however you like
        });
        IMapper mapper = config.CreateMapper();
        return services.AddSingleton(mapper);
    }

This is what YourProfile should look like:

    public YourProfile()
    {
        CreateMap<Person, PersonDTO>(MemberList.Destination)
            .EqualityComparison((p, dto) => p.Id == dto.Id)
            .ReverseMap();

        CreateMap<Customer, CustomerDTO>(MemberList.Destination)
            .ReverseMap();
    }

I have a similar object-graph an this works fine for me.

EDIT I use LazyLoading, if you don´t you have to explicitly load navigationProperties/Collections.