c# - EF Core 关系查询

Question

我是 Entity Framework 的新手，我的表关系看起来有点像这样

public class Customer {
    public int Id { get; set; }
    public string Name { get; set; }

    public List<Product> Products { get; set; }
}

public class Product {
    public int Id { get; set; }

    public int CustomerId { get; set; }
    public Customer Customer { get; set; }

}

我想查询 Customer 表并只包含最后创建的产品 MAX(Id)

普通的 SQL 查询看起来像这样

SELECT * 
FROM Customer
INNER JOIN Product ON Customer.Id = Product.CustomerId
WHERE Product.Id = (SELECT MAX(Id) FROM Product WHERE CustomerId = Customers.Id)

我当前的 EF 查询看起来像这样，但它返回所有产品...

List<Customer> customers = _context.Customers
                .Include(c => c.Products)
                .ToList();

我尝试了类似这样的方法，它给了我正确的结果，但是 EF 做了一堆查询，很快我发现这似乎是错误的方法

List<Customer> customers = _context.Customers
                .Select(c => new Customer() {
                    Id = c.Id,
                    Name = c.Name,
                    c.Products = c.Products.Where(d => d.Id == c.Products.Max(max => max.Id)).ToList()
                }).ToList();

我想要一些建议，或者是否有不同的方法来实现它。

Answer 1

看起来下面的查询可以用不同的方式编写

SELECT * 
FROM Customer
INNER JOIN Product ON Customer.Id = Product.CustomerId
WHERE Product.Id = (SELECT MAX(Id) FROM Product WHERE CustomerId = Customers.Id)   

这可以写成

SELECT TOP 1 *
FROM Customer
INNER JOIN Product ON Customer.Id = Product.CustomerId
Order by Product.Id desc

假设客户名称是必需的，上面的查询可以用 LINQ 或使用 EF 编写，如下所示

var customers = _context.Customers.Join(_context.Products, cu => cu.id,
p => p.CustomerId, (cu,p) => new { cu,p})
.Select( c => new { prodId = c.p.Id,customername = c.cu.Name })
.OrderByDescending( c => c.prodId).Take(1);