在context.SaveChanges（）之前获取新创建的实体的数据库ID（IDENTITY COLUMN）。

Question

Is it possible to reserve an IDENTITY value for a to-be-added row in the database using Entity Framework?

In my override of ObjectContext.SaveChanges , I want to do something like this:

public override int SaveChanges(SaveOptions options)
{
    var affectedEntities = ObjectStateManager.GetObjectStateEntries(
        EntityState.Added |
        EntityState.Modified |
        EntityState.Deleted);

    foreach (var entry in affectedEntities)
    {
        var type = GetObjectType(entry.Entity.GetType());

        if (entry.State == EntityState.Added)
        {
            // log the creation of the new entity into a log file
            // or a different table. For this, I need the newly
            // created entity's ID. Is it possible?
        }
    }

    return base.SaveChanges(options);

    // if I check for ObjectState here, i.e. after SaveChanges, 
    // it won't work? Because once the changes are committed to the database
    // the newly added object's state would no longer remain
    // Added.
}

Answer 1

You can't ask the database to preallocate the id's. You have 2 options

1. implement your own identity server
2. get the id's after the data has been written to the database

Here's a couple of examples of option 2. They both do the same thing - build a list of added items before the call to SaveChanges() and iterate over them after.

public interface IIDentifiable
{
    int id { get; set; }
}

public override int SaveChanges()
{
    ChangeTracker.DetectChanges();
    List<IIDentifiable> addedObjects = GetNewEntries();
    int result = base.SaveChanges();
    foreach (IIDentifiable o in addedObjects)
    {
        //log o.id
    }
    return result;
}

private List<IIDentifiable> GetNewEntries()
{
    var result = (
        from e in ChangeTracker.Entries()
        where e.State == System.Data.EntityState.Added
        select e.Entity)
        .OfType<IIDentifiable>()
        .ToList();

    return result;
}

the drawback of IIDentifiable is getting other details such as Type etc.

The second option is to do the same thing but use dynamic instead of IIDentifiable

public override int SaveChanges()
{
    ChangeTracker.DetectChanges();
    List<dynamic> addedObjects = GetNewEntries();
    int result = base.SaveChanges();
    foreach (dynamic o in addedObjects)
    {
        //log o.id
        //log o.GetType()
    }
    return result;
}

private List<dynamic> GetNewEntries()
{
    var result = (
        from e in ChangeTracker.Entries()
        where e.State == System.Data.EntityState.Added
        select e.Entity)
        .ToList();

    return result;
}