常规设置

要使扩展正常工作，您需要允许 $ expand 查询选项。像这样明确配置：

General Setup

To make expansions work, you need to allow for the $expand query option to be used. Configure it explicitly like so:

private IEdmModel GetEdmModel()
{
    var builder = new ODataConventionModelBuilder();

    builder.EntitySet<EstimateDto>(nameof(MyContext.Estimates))
        .EntityType
        .Expand(); // <-- allow expansion

    builder.EntitySet<TypeDto>(nameof(MyContext.Types))
        .EntityType
        .Expand(); // <-- allow expansion

    builder.EntitySet<SubTypeDto>(nameof(MyContext.SubTypes));
    
    return builder.GetEdmModel();
}

您还需要更新AutoMapper映射，以使查询成功映射到DTO：

You will also need to update your AutoMapper map, to allow the queries to be successfully mapped to a DTOs:

public class AutoMapperConfig : Profile
{
    public AutoMapperConfig()
    {
        CreateMap<Estimate, EstimateDto>()
            .ForMember(
                dto => dto.Types,
                opt => opt.MapFrom(x => x.EstimateTypes.Select(y => y.Type)));

        // The following mapping is needed for expansion to work:
        CreateMap<EstimateTypeRel, TypeDto>()
            .ForMember(
                dto => dto.SubTypes,
                opt => opt.MapFrom(x => x.Type));

        CreateMap<Type, TypeDto>()
            .ForMember(
                dto => dto.SubTypes,
                opt => opt.MapFrom(x => x.SubTypes.Select(y => y.SubType)));

        CreateMap<SubTypeRel, SubTypeDto>();
    }
}

设置该配置后，至少有两种可能的解决方案问题，具体取决于您的要求：

With that configuration being setup, there are at least two possible solutions to this issue, depending on your requirements:

如果仅要扩展 Types ，您需要通过添加来更改AutoMapper映射。where（z => z！= null）子句，因为异常告诉您，集合中不允许 null 值，但是OData包含未扩展的 SubType的值实体：

If you only want to expand Types, you will need to change your AutoMapper mappings by adding a .Where(z => z != null) clause, because as the exception tells you, null values are not allowed in collections, but OData includes them for the non-expanded SubType entities:

public class AutoMapperConfig : Profile
{
    public AutoMapperConfig()
    {
        CreateMap<Estimate, EstimateDto>()
            .ForMember(
                dto => dto.Types,
                opt => opt.MapFrom(
                    x => x.EstimateTypes.Select(y => y.Type)
                        .Where(z => z != null))); // <-- filter out null values

        CreateMap<EstimateTypeRel, TypeDto>()
            .ForMember(
                dto => dto.SubTypes,
                opt => opt.MapFrom(x => x.Type));

        CreateMap<Type, TypeDto>()
            .ForMember(
                dto => dto.SubTypes,
                opt => opt.MapFrom(
                    x => x.SubTypes.Select(y => y.SubType)
                        .Where(z => z != null))); // <-- filter out null values

        CreateMap<SubTypeRel, SubTypeDto>();
    }
}

然后您可以使用以下查询：

Then you can use the following query:

https://localhost:5001/odata/Estimates(1)?$expand=Types

B）同时扩展 SubTypes

另一种方法是扩展 SubTypes 属性，因此可以正确填充集合。要将DTO映射的属性扩展到多个级别，请在查询字符串中使用 $ expand 查询选项，如下所示：

B) Also expand SubTypes

The alternative is to expand the SubTypes property as well, so the collection can be properly filled. To expand DTO mapped properties over multiple levels, use the $expand query option in your query string like so:

https://localhost:5001/odata/Estimates(1)?$expand=Types($expand=SubTypes)