[英]Error while deleting a record with LINQ
I try to change a SQL Delete command into LINQ. 我尝试将SQL Delete命令更改为LINQ。 this is my SQL Command: 这是我的SQL命令:
DELETE FROM [TrackPoints]
WHERE [RouteFK] IN (SELECT RouteId
FROM Routes
WHERE UserId = @UserId)
and this is the LINQ code that I've written: 这是我编写的LINQ代码:
int UID =1;
FirstDataContext aspnetdb = new FirstDataContext();
var res1 = from q1 in aspnetdb.Routes
where q1.UserId == UID
select q1.RouteId;
foreach (var k in res1)
{
var eigen = from p in aspnetdb.Trackpoints
where p.RouteFK == k
select p.TrackPointId;
aspnetdb.Trackpoints.DeleteOnSubmit(eigen.First());
aspnetdb.SubmitChanges();
}
but in this line I have an error: 但在这一行中我有一个错误:
aspnetdb.Trackpoints.DeleteOnSubmit(eigen.First());
that said: 那说:
The best overloaded method match for 'System.Data.Linq.Table.DeleteOnSubmit(LINQ_Test.Trackpoint)' has some invalid arguments 'System.Data.Linq.Table.DeleteOnSubmit(LINQ_Test.Trackpoint)'的最佳重载方法匹配具有一些无效的参数
What should I do? 我该怎么办?
Use this: 用这个:
var eigen = (from p in aspnetdb.Trackpoints
where p.RouteFK == k
select p).First();
or what is shorter: 或更短:
var eigen = aspnetdb.Trackpoints.First(p => p.RouteFK == k);
because it returns Trackpoint
因为它返回Trackpoint
While your select p.TrackPointId
returns Int
. 当您select p.TrackPointId
返回Int
。
var r = from r in aspnetdb.Routes
join p in aspnetdb.Trackpoints on p.RouteId equals r.RouteFK
where r.UserId == UID
select p;
foreach (var x in r)
{
aspnetdb.Trackpoints.DeleteOnSubmit(x);
}
aspnetdb.SubmitChanges();
The problem is that you're just selecting the ID, which then doesn't match the method signature. 问题在于您只是选择ID,然后该ID与方法签名不匹配。 I'd actually change your code to: 我实际上会将您的代码更改为:
var entity = aspnetdb.Trackpoints.Where(p => p.RouteFK == k).Single();
aspnetdb.Trackpoints.DeleteOnSubmit(entity);
(By the time you've change the select
clause to just select p
it becomes pointless to use a query expression - and once you're using the method-call syntax, it makes sense to call Single
or First
in the same statement. As per abatishchev's answer, this could also be aspnetdb.Trackpoints.Single(p => p.RouteFK == k)
.) (当您将select
子句更改为仅select p
时,使用查询表达式就变得毫无意义-并且一旦使用了方法调用语法,就可以在同一条语句中调用Single
或First
。根据abatishchev的回答,这也可能是aspnetdb.Trackpoints.Single(p => p.RouteFK == k)
。)
This assumes you've definitely got 1 (and only 1) entity for the given RouteFK, by the way. 假设您确实为给定的RouteFK拥有1个(只有1个)实体。
It might actually be rather better to select all the entities you want to delete in a single query. 实际上,在单个查询中选择要删除的所有实体可能会更好。 For example: 例如:
var entitiesToDelete = from q1 in aspnetdb.Routes
where q1.UserId == UID
join p in aspnetdb.TrackPoints
on q1.RouteID equals p.RouteFK
select p;
aspnetdb.Trackpoints.DeleteAllOnSubmit(entitiesToDelete);
aspnetdb.SubmitChanges();
Or if you've got your joins set up in your model correctly, you can probably get rid of the explicit join: 或者,如果您在模型中正确设置了联接,则可以摆脱显式联接:
var entitiesToDelete = from q1 in aspnetdb.Routes
where q1.UserId == UID
select q1.Route; // Or Track, or whatever it is
aspnetdb.Trackpoints.DeleteAllOnSubmit(entitiesToDelete);
aspnetdb.SubmitChanges();
In both cases, you can avoid doing nearly as many queries. 在这两种情况下,您都可以避免进行几乎相同数量的查询。 (You avoid an "n+1 selects" problem.) (避免出现“ n + 1个选择”问题。)
The DeleteOnSubmit
method expects the domain object. DeleteOnSubmit
方法需要域对象。 Your code passes an ID
instead. 您的代码改为传递一个ID
。 Also, you lack a condition to check whether the item actually exists. 另外,您没有条件检查项目是否实际存在。
Try to rewrite your routine to something like: 尝试将例程改写为:
foreach (var k in res1)
{
var eigen = from p in aspnetdb.Trackpoints
where p.RouteFK == k
select p;
var item = eigen.FirstOrDefault();
if ( item != null )
{
aspnetdb.Trackpoints.DeleteOnSubmit(item);
aspnetdb.SubmitChanges();
}
}
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