LINQ抱怨子查询返回了超过1行

Question

Simple code that pulls a single record ( it's single because the primary key is an int set as an autonumber ):

using (var db = new DataClasses1DataContext())
{
    var record = db.Projects.Single(x => x.ProjectID == projectID);
    record.Deleted = true;
    record.DeletedByUserID = MySession.Current.UserID;
    record.DeletedOn = DateTime.Now;
    db.SubmitChanges();
}

I'm not sure why but in some cases as soon as it hits db.SubmitChanges() we get an exception stating that Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression. Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

But when I "Watch" the record I only am getting back one record. What could be causing this?

Answer 1

That thing

db.Projects.Single()

means that should be 100% only 1 value to return and there are many.

In that case use

db.Projects.FirstOrDefault()

Or what is better check your data or select conditions.

Enumerable.Single Method (IEnumerable)

And about this ... "But when I "Watch" the record I only am getting back one record. What could be causing this?"

Just check your table to see if you have duplicated entries or if you populate in other way your data context use db.Projects() to get all records and check it.

Answer 2

It may help to use db.Projects.First(x => x.ProjectID == projectID); instead of Single.

It even seems to be a common problem: http://social.msdn.microsoft.com/Forums/en-US/afb292ca-3a61-4a60-8e18-830b92489016/method-single-not-supported-by-linq-to-entities?forum=adodotnetentityframework

Way to solve, don't know if it helps but it's from above link:

    public static TElement Single<TElement>
      (this IQueryable<TElement> query)
{
    if (query.Count() != 1)
    {
        throw new InvalidOperationException();
    }
    return query.First();
}



// Use it like this

Product prod = (from p in db.Product
                where p.ProductID == 711
                select p).Single();