Linq to SQL: Queries don't look at pending cha

问题:

Follow up to this question. I have the following code:

string[] names = new[] { "Bob", "bob", "BoB" };
using (MyDataContext dataContext = new MyDataContext())
{
    foreach (var name in names)
    {
        string s = name;
        if (dataContext.Users.SingleOrDefault(u => u.Name.ToUpper() == s.ToUpper()) == null)
            dataContext.Users.InsertOnSubmit(new User { Name = name });
    }

    dataContext.SubmitChanges();
}

...and it inserts all three names ("Bob", "bob" and "BoB"). If this was Linq-to-Objects, it wouldn't.

Can I make it look at the pending changes as well as what's already in the table?

回答1:

I don't think that would be possible in general. Imagine you made a query like this:

dataContext.Users.InsertOnSubmit(new User { GroupId = 1 });
var groups = dataContext.Groups.Where(grp => grp.Users.Any());

The database knows nothing about the new user (yet) because the insert wasn't commited yet, so the generated SQL query might not return the Group with Id = 1. The only way the DataContext could take into account the not-yet-submitted insert in cases like this would be to get the whole Groups-Table (and possibly more tables, if they are affected by the query) and perform the query on the client, which is of course undesirable. I guess the L2S designers decided that it would be counterintuitive if some queries took not-yet-committed inserts into account while others wouldn't, so they chose to never take them into account.

Why don't you use something like

foreach (var name in names.Distinct(StringComparer.InvariantCultureIgnoreCase))

to filter out duplicate names before hitting the database?

回答2:

Why dont you try something like this

foreach (var name in names)
{
    string s = name;
    if (dataContext.Users.SingleOrDefault(u => u.Name.ToUpper() == s.ToUpper()) == null)
    {
        dataContext.Users.InsertOnSubmit(new User { Name = name });
        break;
    }
}

回答3:

I am sorry, I don't understand LINQ to SQL as much.

But, when I look at the code, it seems you are telling it to insert all the records at once (similar to a transaction) using SubmitChanges and you are trying to check the existence of it from the DB, when the records are not inserted at all.

EDIT: Try putting the SubmitChanges inside the loop and see that the code will run as per your expectation.

回答4:

You can query the appropriate ChangeSet collection, such as

if(
    dataContext.Users.
        Union(dataContext.GetChangeSet().Inserts).
        Except(dataContext.GetChangeSet().Deletes).
        SingleOrDefault(u => u.Name.ToUpper() == s.ToUpper()) == null)

This will create a union of the values in the Users table and the pending Inserts, and will exclude pending deletes.

Of course, you might want to create a changeSet variable to prevent multiple calls to the GetChangeSet function, and you may need to appropriately cast the object in the collection to the appropriate type. In the Inserts and Deletes collections, you may want to filter it with something like

...GetChangeSet().Inserts.Where(o => o.GetType() == typeof(User)).OfType<User>()...