无法使用自动映射器映射内部导航属性。英孚核心

Question

我有针对该特定问题的此类实体：Album、Photo。每个 Album 包含一个集合 Photo。

和 Dto 类: AlbumWithPhotosDto，其中包含集合 PhotoDto。

在我的自动映射器配置文件中，我描述了所有实体的映射规则：

CreateMap<Photo, PhotoDto>()
                .ForMember(dest => dest.AverageRate, opt => opt.MapFrom(src => src.Ratings.Average(r => r.Rate)))
                .ForMember(dest => dest.RatesCount, opt => opt.MapFrom(src => src.Ratings.Count))
                .ReverseMap();

 CreateMap<Album, AlbumWithPhotosDto>()
                .ForMember(dest => dest.PhotosNum, opt => opt.MapFrom(src => src.Photos.Count))
                .ReverseMap();

首先，我使用了 LazyLoadingProxies，当我尝试将 Album 映射到 AlbumWithPhotosDto 时。

我遇到异常：

Error mapping types.

Mapping types: Album -> AlbumWithPhotosDto DAL.Entities.Album -> BLL.DataTransferObjects.AlbumWithPhotosDto

Type Map configuration: Album -> AlbumWithPhotosDto DAL.Entities.Album -> BLL.DataTransferObjects.AlbumWithPhotosDto

Destination Member: Photos

然后我把懒加载改成显式加载如：

_context.Albums
            .Include(a => a.Photos)
            .Include(a => a.Author)

但是我又遇到了同样的异常。我该如何解决这个问题？

这里是类的补充定义：

 public class Album : IEntity<int>
    {
        public int Id { get; set; }
        /// <summary>
        /// Album name.
        /// </summary>
        public string Name { get; set; }
        /// <summary>
        /// Album short description.
        /// </summary>
        public string Description { get; set; }
        /// <summary>
        /// Whether the other users can view and rate the album photos or not.
        /// </summary>
        public bool IsPrivate { get; set; }
        /// <summary>
        /// Foreign key, creator's id.
        /// </summary>
        public string AuthorId { get; set; }
        /// <summary>
        /// User who created the album but not obviously author of all photos.
        /// </summary>
        public virtual User Author { get; set; }
        /// <summary>
        /// Photos in album.
        /// </summary>
        public virtual ICollection<Photo> Photos { get; set; } = new HashSet<Photo>();
    }

public class Photo : IEntity<int>
    {
        public int Id { get; set; }
        /// <summary>
        /// Optional photo short description.
        /// </summary>
        public string Description { get; set; }
        /// <summary>
        /// Path to the photo file on the hard drive.
        /// </summary>
        public string FileName { get; set; }
        /// <summary>
        /// Date and time of uploading.
        /// </summary>
        public DateTime UploadDate { get; set; }
        /// <summary>
        /// Photo ratings by users.
        /// </summary>
        public virtual ICollection<Rating> Ratings { get; set; } = new HashSet<Rating>();
        /// <summary>
        /// Foreigh key, author's id.
        /// </summary> 
        public string AuthorId { get; set; }
        public virtual User Author { get; set; }
        public int AlbumId { get; set; }
        public virtual Album Album { get; set; }
    }

public class AlbumWithPhotosDto : IDtoMetadata
    {
        public int Id { get; set; }
        public string Description { get; set; }
        public string AuthorId { get; set; }
        public int PhotosNum { get; set; }
        public ICollection<PhotoDto> Photos { get; set; }
    }

public class PhotoDto : IDtoMetadata
    {
        public int Id { get; set; }
        public string Base64 { get; set; }
        public string Description { get; set; }
        public double AverageRate { get; set; }
        public int RatesCount { get; set; }
        public DateTime UploadDate { get; set; }
        public string AuthorId { get; set; }
        public ICollection<RatingDto> Ratings { get; set; }
    }

Answer 1

您没有在查询中包含 Ratings。因此，AutoMapper 基本上是在尝试在空集合上调用 .Average()，因此失败。

将您的查询修改为 -

var albums = _context.Albums
            .Include(a => a.Photos).ThenInclude(p => p.Ratings)
            .Include(a => a.Author)
            .ToList();

Answer 2

如异常消息中所述，问题出在 'AlbumWithPhotosDto' 中的 'Photos' 属性。将类型更改为 'List'，然后尝试这种方式。

Answer 3

经过一步一步的投入，发现问题还是出在mapping config的那一行：

CreateMap<Photo, PhotoDto>()
                .ForMember(dest => dest.AverageRate, opt => opt.MapFrom(src => src.Ratings.Average(r => r.Rate)))

解决方案是在评分为 0 时注意默认值。

CreateMap<Photo, PhotoDto>()
                .ForMember(dest => dest.AverageRate, opt => opt.MapFrom(src => src.Ratings.Select(r => r.Rate).DefaultIfEmpty(0).Average()))

现在连 LazyLoadingProxies 都成功映射了，我之前认为它与自动映射器不兼容。

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