在调用 SaveChanges() 之前添加实体的访问 id

Question

i'm using EF 4.3.1, I've overridden the SaveChanges() on the context so that I can get a list of the objects and their states and create entries in my audit log table. I need to store the id of the record in the audit log table so i have a reference to it. This is a problem when records are inserted, as I don't have access to the id before it's saved. Is there any way of getting the id at that point?

public override int SaveChanges()
{
        ChangeTracker.DetectChanges();

        var objectStateManager = ((IObjectContextAdapter)this).ObjectContext.ObjectStateManager;
        var modifiedAuditableEntities = objectStateManager.GetObjectStateEntries(EntityState.Modified | EntityState.Added).Where(e => (IAuditable)e.Entity != null);

        foreach (var entry in modifiedAuditableEntities)
        {
            var entity = (IAuditable)entry.Entity;

            if (entity != null)
            {
                switch (entry.State)
                {
                    case EntityState.Added:
                        entity.IsAdded = true;
                        break;
                    case EntityState.Deleted:
                        entity.IsDeleted = true;
                        break;
                    case EntityState.Modified:
                        entity.IsModified = true;
                        break;
                }

                this.EntitySet<AuditLogEntry>().Add(this.auditLogService.CreateAuditLogEntryForEntity((IAuditable)entry.Entity));
            }
        return base.SaveChanges();
}

Answer 1

If your goal is that you want both your save, and your audit log to be created at the same time, you could wrap it in a transaction scope so your method to be atomic.

    public override int SaveChanges()
    {
        ChangeTracker.DetectChanges();
        using (var scope = new TransactionScope())
        {
            var objectStateManager = ((IObjectContextAdapter) this).ObjectContext.ObjectStateManager;
            var modifiedAuditableEntities =
                objectStateManager.GetObjectStateEntries(EntityState.Modified | EntityState.Added).Where(
                    e => (IAuditable) e.Entity != null);
            var result = base.SaveChanges();
            foreach (var entry in modifiedAuditableEntities)
            {
                var entity = (IAuditable) entry.Entity;

                if (entity != null)
                {
                    switch (entry.State)
                    {
                        case EntityState.Added:
                            entity.IsAdded = true;
                            break;
                        case EntityState.Deleted:
                            entity.IsDeleted = true;
                            break;
                        case EntityState.Modified:
                            entity.IsModified = true;
                            break;
                    }

                    this.EntitySet<AuditLogEntry>().Add(
                        this.auditLogService.CreateAuditLogEntryForEntity((IAuditable) entry.Entity));
                }
            }
            base.SaveChanges();

            scope.Complete();
            return result;
        }
    }

Answer 2

Id the Id is an integer generated by the database there isnt any way of getting it before calling save changes. Possible solutions include:

• Triggers
• Using Guids as Primary Keys
• Using a key generation strategy not managed by the database (HiLo in NHibernate)
• Calling base.SaveChanges() first then inspecting the results.

Answer 3

I would recommend using a stored procedure at the database level to perform your insert/auditing functions (and other database changes), and revoking the insert/update/delete permission from your users. As such you can guarantee the integrity of the update process and of the audit table.