无法使用自动映射器映射内部导航属性。 EF 核心

Question

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

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

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LazyLoadingProxies，当我尝试将Album映射到AlbumWithPhotosDto时。

我遇到了异常：

错误映射类型。

映射类型：相册 -> AlbumWithPhotosDto DAL.Entities.Album -> BLL.DataTransferObjects.AlbumWithPhotosDto

类型映射配置：相册 -> AlbumWithPhotosDto DAL.Entities.Album -> BLL.DataTransferObjects.AlbumWithPhotosDto

目标成员：照片

然后我在显式加载时更改了延迟加载，例如：

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

但我又遇到了同样的例外。我该如何解决这个问题？

这里是类的附加定义：

 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

您的查询中没有包含Ratings。因此，AutoMapper 基本上是试图在一个空集合上调用.Average()，因此失败了。

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

正如异常消息中所述，问题在于“AlbumWithPhotosDto”中的“照片”属性。将类型更改为“列表”，然后尝试这种方式。

Answer 3

经过一步一步的投资，我意识到问题出在映射配置中的那一行：

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()))

现在连我之前认为与自动映射器不兼容的 LazyLoadingProxies 都已成功映射。