Question

当我尝试在 where 子句中使用 props.getValue 方法时，出现错误。出现这个错误是因为LINQ无法在SQL中翻译这个方法的调用，因为SQL不知道这个方法。我读过一些关于表达式树的内容，但我不太明白如何使用它们，所以如果有人能解释我如何正确使用它们就太好了。

我的控制器方法：

  int end = pageNumber * filter; int start = end - filter;
            var query = GetRequestsWithoutFilter().Where(r => r.RequestStatus == 1);

            if (attribute != null && title!=null)
            {
                if (attribute.Equals("movieReleaseDate"))
                {
                    int i = Int32.Parse(title);
                    query = query.Where(r => (r.Movie.MovieReleaseDate) == i);
                }
                else
                {
                    Movie m = new Movie();
                    PropertyInfo props = m.GetType().GetProperties().Where(p => String.Equals(attribute, p.Name, StringComparison.OrdinalIgnoreCase)).FirstOrDefault();

                    if (props!=null)
                    {
                        query = query.Where(r => ((string)props.GetValue(r.Movie)).Substring(0, (title.Length < r.Movie.MovieTitle.Length ? title.Length : r.Movie.MovieTitle.Length)).ToUpper().Equals(title));
                    }
                }
            }

            try { 
             var result = await query.Skip(start).Take(filter).ToListAsync();
                if (result == null)
                {
                    return NotFound();
                }
                return result;
            } catch(Exception e)
            {
                return null;
            }
            
        }

GetRequestsWithoutFilter:

 [NonAction]
        private IQueryable<Request> GetRequestsWithoutFilter()
        {
            IQueryable<Request> query = _context.Requests.Include(r => r.User)
                                                .Include(r => r.Movie)
                                                .Select(e => new Request()
                                                {
                                                    RequestID = e.RequestID,
                                                    MovieID = e.MovieID,
                                                    Movie = new Movie()
                                                    {
                                                        MovieID = e.MovieID,
                                                        MovieTitle = e.Movie.MovieTitle,
                                                        MovieDescription = e.Movie.MovieDescription,
                                                        MovieDirector = e.Movie.MovieDirector,
                                                        MovieProducer = e.Movie.MovieProducer,
                                                        ImageSrc = String.Format("{0}://{1}{2}/Images/Movies/{3}", Request.Scheme, Request.Host, Request.PathBase, e.Movie.ImageName),
                                                        MovieReleaseDate = e.Movie.MovieReleaseDate, 
                                                    },
                                                    UserID = e.UserID,
                                                    User = new User()
                                                    {
                                                        UserID = e.User.UserID,
                                                        UserVorname = e.User.UserVorname,
                                                        UserNachname = e.User.UserNachname
                                                    },
                                                    RequestDate = e.RequestDate,
                                                    RequestStatus = e.RequestStatus
                                                });
            return query;
        }

电影模型：

using Microsoft.AspNetCore.Http;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Threading.Tasks;

namespace MoviesWebApi.Models
{
    public class Movie
    {
        public Movie()
        {
            Requests = new HashSet<Request>();
        }


        [Key]
        public int MovieID { get; set; }

        [Column(TypeName = "nvarchar(40)")]
        public string MovieTitle { get; set; }
        [Column(TypeName = "nvarchar(1000)")]
        public string MovieDescription { get; set; }
        [Column(TypeName = "nvarchar(40)")]
        public string MovieDirector { get; set; }
        [Column(TypeName = "nvarchar(40)")]
        public string MovieProducer { get; set; }

        public int MovieReleaseDate { get; set; }

        [Column(TypeName = "nvarchar(100)")]
        public string ImageName { get; set; }

        [NotMapped]
        public IFormFile ImageFile { get; set; }

        [NotMapped]
        public string ImageSrc { get; set; }

        public  ICollection<Request> Requests { get; set; }

    }
}

Answer 1

像这样的反射调用

((string)props.GetValue(r.Movie))

作为查询表达式树的一部分不可翻译。

您在此处获得的是检索属性的值。与它等价的表达式是 Expression.Property，但是您需要使用 Expression 类方法构造整个谓词，这既乏味又容易出错，或者使用一些辅助实用程序来组合表达式。

这并不难，但在 EF Core 中有一个更简单的解决方案，因为它提供了一种称为 EF.Property 的特殊方法，它可以被识别并正确翻译。

为了使用它，您只需要知道属性名称和类型。后者在动态场景中有点问题，但幸运的是您似乎针对 string 类型属性，因此只需将上面的替换为

EF.Property<string>(r.Movie, props.Name)

.Net核心实体框架LINQ表达式无法翻译

1 个答案: