如何使用linq獲取相關數據

Question

我正在尋找一些更智能的方式來使用IQueryable獲取相關的圖書描述。

IQueryable list = db.Students.Select(x => new StudentsViewModel 
     { 
      StudentID = x.StudentID, 
      Name = x.Name, 
      BookID = x.BookID, 
      DescriptionForBookByBookID = db.Books.Where(v => v.BookID == x.BookID).Select(v => v.Description).FirstOrDefault().ToString() 
     }); 

感謝您的任何意見;）

Answer 1

定義導航屬性爲Book在Student實體：

public int StudentID { get; set; } 
public string Name { get; set; } 
public int? BookID { get; set; } // assume book is optional, but you can use int 
public virtual Book Book { get; set; } 

並用它來獲得本書描述爲視圖模式：

db.Students.Select(s => new StudentsViewModel 
{ 
    StudentID = s.StudentID, 
    Name = s.Name, 
    BookID = s.BookID, 
    DescriptionForBookByBookID = s.Book.Description 
}) 

生成的查詢看起來像

SELECT 
    [Extent1].[StudentID] AS [StudentID], 
    [Extent1].[Name] AS [Name], 
    [Extent1].[BookID] AS [BookID], 
    [Extent2].[Description] AS [Description] 
    FROM [dbo].[Students] AS [Extent1] 
    LEFT OUTER JOIN [dbo].[Books] AS [Extent2] 
     ON [Extent1].[BookID] = [Extent2].[BookID]