EntityFramework Codefirst Select 查询指定列

本笔记解决如下两个问题：

1、查询返回指定列。

2、查询指定列时报错如下：

The entity or complex type 'DataBase.AccordionModels' cannot be constructed in a LINQ to Entities query.

 

一、环境中类定义如下：
DataBaseContext,

  public DataBaseContext()
            : base("default")
        {
        public DbSet<UserModels> UserContext { get; set; }
        public DbSet<PigModels> PigContext { get; set; }
        public DbSet<AccordionModels> AccordionContext { get; set; }
        public DbSet<HrefModels> HrefContext { get; set; }
         }

 AccordionModels:

  public  class AccordionModels
    {
       [Required]
       [Key]
       public string ID { get; set; }
        [Required]
        [Display(Name = "名称")]
        public string title { get; set; }
        [Display(Name = "icon")]
        public string icon { get; set; }
        [Display(Name = "顺序号")]
        public int order { get; set; }

        public virtual ICollection<HrefModels> Hrefs { get; set; }
    }

HrefModels:

public class HrefModels
    {
       [Required]
       [Key]
       public string ID { get; set; }
       
        [Required]
        [Display(Name = "图标")]
        public string icon { get; set; }
        [Display(Name = "链接")]
        public string link { get; set; }
        [Display(Name = "名称")]
        public string title { get; set; }
        public int iFrame { get; set; }
        public virtual AccordionModels  Accordion { get; set; }       
       
    }

二、查询方法

（1）报错的查询方法：当采用如下写法查询时，会抛出异常 Message:The entity or complex type 'DataBase.AccordionModels' cannot be constructed in a LINQ to Entities query.

 List<AccordionModels> accordions = new List<AccordionModels>();
                    accordions = context.AccordionContext.Select(a => new AccordionModels()
                    {
                        ID = a.ID,
                        title = a.title,
                        icon = a.icon
                    }).ToList();

 

可用如下两种方法：注意Select中的内容：1、新建一个实体类 AccordionView

 List<AccordionView> accordions = new List<AccordionView>();
                    accordions = context.AccordionContext.Select(a => new AccordionView()
                    {
                        ID = a.ID,
                        title = a.title,
                        icon = a.icon
                    }).ToList();

2、查询之后在转换

List<AccordionModels> AccordionModels = context.AccordionContext.Select(a => new
           {
           a.ID,
         a.title,
          a.icon

       }).ToList()
         .Select(b => new AccordionModels()
         {
         ID = b.icon,
         title = b.title,
        icon = b.icon
         }).ToList();