ef-core-3.1 - 查询多对多ef核心关系
问题描述
我有以下课程
public class Book
{
public string BookId { get; set; }
public string Title { get; set; } = string.Empty;
public string Description { get; set; } = string.Empty;
// Relationships
public ICollection<BookCategory> CategoriesLink { get; set; }
}
public class BookCategory
{
public string BookId { get; set; }
public string CategoryId { get; set; }
public Book Book { get; set; }
public Category Category { get; set; }
}
public class Category
{
public string CategoryId { get; set; }
public string Name { get; set; }
public ICollection<BookCategory> BooksLink { get; set; }
}
我不确定如何获得Book
给定的所有 s Category
。我正在使用 EFCore 3.1。我尝试了以下,但不确定它是否有效。
return context.Categories
.Include(x => x.BooksLink)
.ThenInclude(x => x.Book)
.Where(x => x.CategoryId == category)
.SelectMany(x=>x.BooksLink.Select(y=>y.Book))
.ToList();
解决方案
由于性能原因,不建议使用 .Include 方法。
为了更好的查询,我会使用这个 linq
from book in context.book
join bookCatJoin in context.BookCategory on book.id equals bookCatJoin.BookId
join category in context.Category.Where(e=>e.Name=="crime") on bookCatJoin.categoryId equals category.CategoryId
select book;
此查询的性能比包含更好。因为每个包含都与整个表进行左连接。 https://docs.microsoft.com/en-us/ef/core/querying/related-data .Include 的文档强调了此建议