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
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) }
};
}
}
其他回答
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};
我发现如果你熟悉SQL语法，使用LINQ查询语法会更清晰、更自然，并且更容易发现错误:
var id = 1;
var query =
from post in database.Posts
join meta in database.Post_Metas on post.ID equals meta.Post_ID
where post.ID == id
select new { Post = post, Meta = meta };
如果您真的坚持使用lambdas，那么您的语法就有很大的问题。下面是相同的查询，使用LINQ扩展方法:
var id = 1;
var query = database.Posts // your starting point - table in the "from" statement
.Join(database.Post_Metas, // the source table of the inner join
post => post.ID, // Select the primary key (the first part of the "on" clause in an sql "join" statement)
meta => meta.Post_ID, // Select the foreign key (the second part of the "on" clause)
(post, meta) => new { Post = post, Meta = meta }) // selection
.Where(postAndMeta => postAndMeta.Post.ID == id); // where statement
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) }
};
}
}
因为当我开始LINQ + EntityFramework时，我盯着这些例子看了一天。
如果你正在使用EntityFramework，并且你在Post模型对象上设置了一个名为Meta的导航属性，这非常简单。如果你用的是实体，没有导航属性，还在等什么?
database
.Posts
.Where(post => post.ID == id)
.Select(post => new { post, post.Meta });
如果你先做代码，你可以这样设置属性:
class Post {
[Key]
public int ID {get; set}
public int MetaID { get; set; }
public virtual Meta Meta {get; set;}
}
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。
