Error while deleting a record with LINQ

Question

I try to change a SQL Delete command into LINQ. this is my SQL Command:

DELETE FROM [TrackPoints]
WHERE [RouteFK] IN (SELECT RouteId
                    FROM Routes
                    WHERE UserId = @UserId)

and this is the LINQ code that I've written:

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:

aspnetdb.Trackpoints.DeleteOnSubmit(eigen.First());

that said:

The best overloaded method match for 'System.Data.Linq.Table.DeleteOnSubmit(LINQ_Test.Trackpoint)' has some invalid arguments

What should I do?

Answer 1

Use this:

var eigen = (from p in aspnetdb.Trackpoints
            where p.RouteFK == k
            select p).First();

or what is shorter:

var eigen = aspnetdb.Trackpoints.First(p => p.RouteFK == k);

because it returns Trackpoint

While your select p.TrackPointId returns 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();

Answer 2

The problem is that you're just selecting the ID, which then doesn't match the method signature. I'd actually change your code to:

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) .)

This assumes you've definitely got 1 (and only 1) entity for the given RouteFK, by the way.

It might actually be rather better to select all the entities you want to delete in a single query. For example:

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:

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. (You avoid an "n+1 selects" problem.)

Answer 3

The DeleteOnSubmit method expects the domain object. Your code passes an ID instead. Also, you lack a condition to check whether the item actually exists.

Try to rewrite your routine to something like:

        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();
            }

        }