C# LINQ 嵌套选择查询

Question

我有一个场景如下嵌套

--Orders (List) 
 ----Products (List)
 ------Manufacturers (List) 
       FIELDS 
        -Name
        -Address
        -City 

在这种情况下，我需要执行查询，该查询将过滤制造商城市并返回Orders, Products & only matching city manufacturers

我尝试提出以下查询，但是即使 city 与Manufacturers不匹配，我也得到了所有产品列表。

var filteredOrders = from o in Orders
                    from t in o.Products                           
                    where t.Manufacturers.Any(v => v.City == "Hartford")
                    select o;

或者，即使我从select o“选择 t.Manufacturers”更改为“选择 t.Manufacturers”，我也会获得所有制造商列表，无论城市过滤器如何。

幸运的是，我得到了符合我的场景的 W3school SQL 示例。 https://www.w3schools.com/sql/trysql.asp?filename=trysql_op_or

SQL查询：

SELECT o.OrderId, p.ProductName, s.* 
FROM [Orders] o 
JOIN OrderDetails od ON o.OrderId = od.OrderId AND o.orderId = 10248 
JOIN Products p ON od.ProductId = p.ProductId
JOIN Suppliers s ON p.SupplierId = s.SupplierId and s.City ='Singapore'    

Answer 1

我会将所有内容展平，然后仅过滤您想要的城市：

class Manufacturer
{
    public string Name;
    public string Address;
    public string City;
}

class Product
{
    public Manufacturer[] Manufacturers;
}

class Order
{
    public Product[] Products;
}

static void Main(string[] args)
{
    var cities = new string[] { "a", "b" };
    Order[] orders = null;
    orders.SelectMany(o => o.Products.SelectMany(p => p.Manufacturers.Select(m => new { o, p, m })))
        .Where(g => cities.Contains(g.m.City))
        .ToList();
    }

或者，如果你想返回new Order s （因为它们有不同的Products，它必须指向新分配的Object）你可以这样：

var newOrders = orders.Select(o => new Order()
{
    Products = o.Products
    .Select(p => new Product()
    {
        Manufacturers = p.Manufacturers.Where(m => cities.Contains(m.City)).ToArray()
    })
    .Where(m => m.Manufacturers.Length > 0).ToArray()
}).Where(p => p.Products.Length > 0).ToArray();