c# - EF Core - 无法访问 Select() 中的导航属性

Question

我正在尝试访问 IdentityUser 模型的导航角色属性。

在 GetQueryable 函数中我正在设置 include 属性

protected virtual IQueryable<TEntity> GetQueryable<TEntity>(
    Expression<Func<TEntity, bool>> filter = null,
    string includeProperties = null
    )
    where TEntity : class, IEntity
    {
        IQueryable<TEntity> query = context.Set<TEntity>();
        if (filter != null)
        {
            query = query.Where(filter);
        }

        if (includeProperties != null)
        {
            query = query.Include(includeProperties);
        }
        return query;
    }

如果我执行以下查询，则角色属性已成功填充:

return GetQueryable<ApplicationUser>(e => e.Id == id, "Roles").SingleOrDefault();

但是当我将投影(选择)与以下 Dto 一起使用时:

public class ApplicationUserDto: BaseDto
{
    public string Email { get; set; }
    public string Name { get; set; }
    public List<IdentityUserRole<string>> Roles{ get; set; }

    public static Expression<Func<ApplicationUser, ApplicationUserDto>> SelectProperties = (user) => new ApplicationUserDto {
        Id = user.Id,
        Email = user.Email,
        Name = user.Name,
        Roles = (List<IdentityUserRole<string>>)user.Roles
    };
}

然后下面的查询崩溃了:

return GetQueryable<ApplicationUser>(e => e.Id == id, "Roles").Select(ApplicationUserDto.SelectProperties).SingleOrDefault();

错误如下:

System.InvalidOperationException: The type of navigation property 'Roles' on the entity type 'IdentityUser<string, IdentityUserClaim<string>, IdentityUserRole
<string>, IdentityUserLogin<string>>' is 'ICollection<IdentityUserRole<string>>' for which it was not possible to create a concrete instance. Either initialize the
property before use, add a public parameterless constructor to the type, or use a type which can be assigned a HashSet<> or List<>.

它还会记录一个警告:

warn: Microsoft.EntityFrameworkCore.Query.RelationalQueryCompilationContextFactory[6]
      The Include operation for navigation: 'user.Roles' was ignored because the target navigation is not reachable in the final query results. 

Answer 1

您需要具体化(执行子查询，这将显式收集角色)以便将其分配给您的 DTO:

public class ApplicationUserDto: BaseDto
{
    public string Email { get; set; }
    public string Name { get; set; }
    public List<IdentityUserRole<string>> Roles{ get; set; }

    public static Expression<Func<ApplicationUser, ApplicationUserDto>> SelectProperties = (user) => new ApplicationUserDto {
        Id = user.Id,
        Email = user.Email,
        Name = user.Name,
        Roles = user.Roles.ToList()
    };
}

并记得添加:
使用 System.Linq；
到您的文件，以便能够在 ICollection 上调用 .ToList()。

正如您在评论中所述，您想要的类型是一个字符串，因此您可以在 user.Roles 之后执行任何操作，即像这样执行进一步的投影:

user.Roles.Select(role=> role.RoleId).ToList()

请记住事后具体化结果。