如何使用AutoMapper将嵌套集合属性投影到DTO并返回投影集合？

需求：将源模型LargeEntity的List<ExampleModelContainer>类型属性ModelEntries，投影为目标模型LargeEntityDto的List<ExampleModelDto>类型属性Models，且要求大部分投影操作在EF服务器端完成。

源模型 #

public class ExampleModel
{
    public int Id { get; set; }
    public float Something { get; set; }
    public string Info { get; set; } = "";
}

public class ExampleModelContainer
{
    public int Id { get; set; }
    public ExampleModel Model { get; set; } = null!;
}

public class LargeEntity
{
    public int Id { get; set; }
    public List<ExampleModelContainer> ModelEntries { get; set; } = null!;
}

目标模型 #

public class ExampleModelDto
{
    public float Something { get; set; }
    public string Info { get; set; } = "";
}

public class LargeEntityDto
{
    public int Id { get; set; }
    public List<ExampleModelDto> Models { get; set; } = null!;
}

尝试的配置及异常 #

尝试通过定义ExampleModelContainer到ExampleModel的转换间接映射，但配置验证失败：

public class AutoMapperProfile : Profile
{
    public AutoMapperProfile()
    {
        CreateMap<ExampleModel, ExampleModelDto>();
        
        // 尝试将容器转换为模型，再自动转为DTO，但失败
        CreateMap<ExampleModelContainer, ExampleModel>()
            .ConvertUsing(x => x.Model);
        
        CreateMap<LargeEntity, LargeEntityDto>()
            .ForMember(x => x.Models, 
                opt => opt.MapFrom(y => y.ModelEntries));
    }
}

异常信息：

Unhandled exception. AutoMapper.AutoMapperConfigurationException: The following member on AutoMapperExps.Models.LargeEntityDto cannot be mapped:
        Models
Add a custom mapping expression, ignore, add a custom resolver, or modify the destination type AutoMapperExps.Models.LargeEntityDto.
Context:
        Mapping to member Models from AutoMapperExps.Models.LargeEntity to AutoMapperExps.Models.LargeEntityDto
Exception of type 'AutoMapper.AutoMapperConfigurationException' was thrown.
   at AutoMapper.Configuration.ConfigurationValidator.AssertConfigurationIsValid(IEnumerable`1 typeMaps)
   at AutoMapper.Configuration.ConfigurationValidator.AssertConfigurationExpressionIsValid(IEnumerable`1 typeMaps)
   at AutoMapper.MapperConfiguration.AssertConfigurationIsValid()
   at Program.Main(String[] args) in D:\Experiments\AutoMapperExps\Program.cs:line 1

解决方案 #

直接在MapFrom中通过LINQ选择容器内的Model属性，结合已有的ExampleModel到ExampleModelDto的映射，既满足配置验证，又支持EF服务器端投影：

public class AutoMapperProfile : Profile
{
    public AutoMapperProfile()
    {
        // 定义基础模型到DTO的映射，EF可解析该表达式
        CreateMap<ExampleModel, ExampleModelDto>();

        CreateMap<LargeEntity, LargeEntityDto>()
            .ForMember(dest => dest.Models, 
                // 直接从每个容器中提取Model，AutoMapper会自动应用ExampleModel→ExampleModelDto的映射
                opt => opt.MapFrom(src => src.ModelEntries.Select(container => container.Model)));
    }
}

说明 #

1. 避免自定义转换：之前使用ConvertUsing的方式会生成客户端评估代码，无法被EF转换成SQL，同时AutoMapper无法自动将List<ExampleModelContainer>转换为List<ExampleModelDto>，导致配置验证失败。
2. 服务器端投影支持：上述配置中的Select(container => container.Model)是EF可解析的表达式，结合ExampleModel到ExampleModelDto的映射，AutoMapper会生成完整的服务器端投影SQL，只查询Something和Info等必要字段。
3. 配置验证通过：该配置可以正常通过AssertConfigurationIsValid()验证，无映射错误。

内容的提问来源于stack exchange，提问作者anon