如何在.NET Rest API中按需加载/不加载School的嵌套Student实体？

我拥有一个带控制器的.NET Rest API，该控制器负责处理实体School的请求。数据库中存在多个School，每个School关联多个Student。

有时我希望查询返回包含对应Student的所有School，有时仅需返回School列表。

我尝试在查询中添加布尔参数includeStudents，通过Entity Framework的Include方法修改内部SQL查询以关联Students表，实现代码如下：

public class SchoolsController:ApiController
{
    [System.Web.Http.HttpGet]
    public IEnumerable<SchoolsDTO> Index(int? id = null, bool? includeStudents=null)
    {
        using (MyDatabase db = new MyDatabase())
        {
            string includeString = "";
            if(includeStudents.HasValue && includeStudents.Value)
            {
                includeString = ".Students";
            }
            var schools = db.Schools.Include(includeString).AsQueryable();
            if (id.HasValue)
            {
                schools = schools.Where(x => x.Id == id);
            }
           
            return AutoMapper.Mapper.Map<List<SchoolsDTO>>(schools.ToList());
        }
    }
}

但无论是否使用Include方法，查询始终会解析并序列化所有School的Student数据到JSON中。

补充说明：我使用AutoMapper处理DTO，但这些实体的DTO是EF原始对象的完全副本。

问题根源在于EF延迟加载自动触发和AutoMapper默认全量映射，可以通过以下步骤解决：

1. 关闭EF延迟加载与代理创建
   EF默认开启延迟加载，即便未调用Include，序列化器访问School.Students属性时，EF会自动查询数据库加载关联数据。在数据库上下文构造函数中关闭该功能：

public class MyDatabase : DbContext
{
    public MyDatabase()
    {
        this.Configuration.LazyLoadingEnabled = false;
        this.Configuration.ProxyCreationEnabled = false; // 禁用动态代理，避免序列化时的额外问题
    }

    // DbSet<School> Schools { get; set; }
    // DbSet<Student> Students { get; set; }
}

2. 优化查询逻辑，条件添加Include
   原代码中Include("")属于无效调用，改为条件判断是否添加关联查询，同时使用强类型Include避免字符串拼写错误：

var schoolsQuery = db.Schools.AsQueryable();
if(includeStudents.HasValue && includeStudents.Value)
{
    schoolsQuery = schoolsQuery.Include(s => s.Students);
}
if(id.HasValue)
{
    schoolsQuery = schoolsQuery.Where(s => s.Id == id);
}
var schoolsList = schoolsQuery.ToList();

3. 控制AutoMapper的映射规则
   由于DTO是EF实体的完全副本，AutoMapper默认会映射所有属性，需要根据参数决定是否映射Students：

• 方式一：映射时动态忽略属性

if(includeStudents.HasValue && includeStudents.Value)
{
    return AutoMapper.Mapper.Map<List<SchoolsDTO>>(schoolsList);
}
else
{
    return AutoMapper.Mapper.Map<List<SchoolsDTO>>(schoolsList, opt => 
        opt.ForMember(dest => dest.Students, opt => opt.Ignore())
    );
}

• 方式二：提前定义两套映射配置

// 初始化映射
AutoMapper.Mapper.Initialize(cfg => {
    // 不包含Students的映射
    cfg.CreateMap<School, SchoolsDTO>();
    // 包含Students的映射
    cfg.CreateMap<School, SchoolsDTO>()
        .ForMember(dest => dest.Students, opt => opt.MapFrom(src => src.Students))
        .Named("IncludeStudents");
});

// 控制器中调用对应映射
if(includeStudents.HasValue && includeStudents.Value)
{
    return AutoMapper.Mapper.Map<List<SchoolsDTO>>(schoolsList, opt => opt.UseNamedProfile("IncludeStudents"));
}
else
{
    return AutoMapper.Mapper.Map<List<SchoolsDTO>>(schoolsList);
}

内容的提问来源于stack exchange，提问作者E. Williams