Question

我正在尝试访问IdentityUser模型的导航Roles属性。

在GetQueryable函数内部我设置了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，则会成功填充roles属性：

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

但是当我使用跟随Dto的投影（选择）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
    };
}

然后以下查询崩溃：

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

出现以下错误：

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

您需要实现（执行子查询，它将显式收集角色），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()
    };
}

请记得添加： using System.Linq; 到您的文件，以便能够在.ToList()上致电ICollection。

正如你在评论中所说，你想要的类型是一个字符串，所以你可以在user.Roles之后做任何事情，即进行如下的进一步投影：

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

请记住事后实现结果。

Answer 2

实际上，如果我们使用Include，则会忽略EF Core 1.1 Select()，请参阅以下内容：

忽略包含部分： https://docs.microsoft.com/en-us/ef/core/querying/related-data

这就是它显示警告的原因：

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.

和ToList()也无效。因此，导航属性需要另一个投影才能工作。

    public static Expression<Func<ApplicationRole, ApplicationRoleDto>> SelectProperties = (role) => new ApplicationRoleDto
    {
        Id = role.Id,
        Name = role.Name.Substring(0, role.Name.Length - role.TenantId.Length),
        Description = role.Description,
        // another projection on Claims navigation property
        Claims = role.Claims.Select(claim => claim.ClaimValue).ToList(),
    };

注意：在这种情况下，它们也是性能问题，因为如果我们在列表上使用Select（），则不会发生急切加载（因为忽略了Include（））它会生成一个单独的sql查询以获取结果集中每个元素的导航属性。

另一方面，如果我们仅使用Include()，它会按预期进行连接，因此效果会更好。

EF Core - 无法访问Select（）中的导航属性

2 个答案: