我遇到了用LINQ和Lambda编写的查询的麻烦。到目前为止，我得到了很多错误，这是我的代码:
int id = 1;
var query = database.Posts.Join(database.Post_Metas,
post => database.Posts.Where(x => x.ID == id),
meta => database.Post_Metas.Where(x => x.Post_ID == id),
(post, meta) => new { Post = post, Meta = meta });
我不确定这个问题是否正确。
当前回答
Daniel很好地解释了语法关系，但是为了让我的团队更容易理解，我把这个文档放在一起。希望这对大家有所帮助
其他回答
Daniel很好地解释了语法关系，但是为了让我的团队更容易理解，我把这个文档放在一起。希望这对大家有所帮助
LINQ连接的查询语法
var productOrderQuery = from product in Product.Setup()//outer sequence
join order in OrderDetails.Setup()//inner sequence
on product.Id equals order.ProductId //key selector
select new//result selector
{
OrderId = order.Id,
ProductId = product.Id,
PurchaseDate = order.PurchaseDate,
ProductName = product.Name,
ProductPrice = product.Price
};
LINQ连接的方法语法
var productOrderMethod = Product.Setup().//outer sequence
Join(OrderDetails.Setup(), //inner sequence
product => product.Id//key selector
,order=> order.ProductId //key selector
,(product,order)=> //projection result
new
{
OrderId = order.Id,
ProductId = product.Id,
PurchaseDate = order.PurchaseDate,
ProductName = product.Name,
ProductPrice = product.Price
}
);
Product.cs供参考
class Product
{
public int Id { get; set; }
public string Name { get; set; }
public decimal Price { get; set; }
public static IEnumerable<Product> Setup()
{
return new List<Product>()
{
new Product(){Id=1, Name="Bike", Price=30.33M },
new Product(){Id=2, Name="Car", Price=50.33M },
new Product(){Id=3, Name="Bus", Price=60.33M }
};
}
}
cs类，以供参考
class OrderDetails
{
public int Id { get; set; }
public virtual int ProductId { get; set; }
public DateTime PurchaseDate { get; set; }
public static IEnumerable<OrderDetails> Setup()
{
return new List<OrderDetails>()
{
new OrderDetails(){Id=1, ProductId=1, PurchaseDate= DateTime.Now },
new OrderDetails(){Id=2, ProductId=1, PurchaseDate=DateTime.Now.AddDays(-1) },
new OrderDetails(){Id=3, ProductId=2, PurchaseDate=DateTime.Now.AddDays(-2) }
};
}
}
可能是这样的
var myvar = from a in context.MyEntity
join b in context.MyEntity2 on a.key equals b.key
select new { prop1 = a.prop1, prop2= b.prop1};
1 = 1两个不同的表连接
var query = from post in database.Posts
join meta in database.Post_Metas on 1 equals 1
where post.ID == id
select new { Post = post, Meta = meta };
您的键选择器不正确。它们应该接受与所讨论的表类型相同的对象，并返回在连接中使用的键。我想你的意思是:
var query = database.Posts.Join(database.Post_Metas,
post => post.ID,
meta => meta.Post_ID,
(post, meta) => new { Post = post, Meta = meta });
您可以随后应用where子句，而不是作为键选择器的一部分。
