Entity Framework/SQL2008 - How to Automatically Update LastModified fields for Entities?

Question

If i have the following entity:

public class PocoWithDates {    public string PocoName { get; set; }    public DateTime CreatedOn { get; set; }    public DateTime LastModified { get; set; } } 

Which corresponds to a SQL Server 2008 table with the same name/attributes...

How can i automatically:

1. Set the CreatedOn/LastModified field for the record to now (when doing INSERT)
2. Set the LastModified field for the record to now (when doing UPDATE)

When i say automatically, i mean i want to be able to do this:

poco.Name = "Changing the name"; repository.Save();  

Not this:

poco.Name = "Changing the name"; poco.LastModified = DateTime.Now; repository.Save(); 

Behind the scenes, "something" should automatically update the datetime fields. What is that "something"?

I'm using Entity Framework 4.0 - is there a way that EF can do that automatically for me? (a special setting in the EDMX maybe?)

From the SQL Server side, i can use DefaultValue, but that will only work for INSERT's (not UPDATE's).

Similarly, i can set a default value using a constructor on the POCO's, but again this will only work when instantiating the object.

And of course i could use Triggers, but it's not ideal.

Because i'm using Entity Framework, i can hook into the SavingChanges event and update the date fields here, but the problem is i need to become "aware" of the POCO's (at the moment, my repository is implemented with generics). I would need to do some sort of OO trickery (like make my POCO's implement an interface, and call a method on that). I'm not adversed to that, but if i have to do that, i would rather manually set the fields.

I'm basically looking for a SQL Server 2008 or Entity Framework 4.0 solution. (or a smart .NET way)

Any ideas?

EDIT

Thanks to @marc_s for his answer, but i went with a solution which is better for my scenario.

Answer 1

I know I'm a little late to the party, but I just solved this for a project I'm working on and thought I'd share my solution.

First, to make the solution more re-usable, I created a base class with the timestamp properties:

public class EntityBase {     public DateTime? CreatedDate { get; set; }     public DateTime? LastModifiedDate { get; set; } } 

Then I overrode the SaveChanges method on my DbContext:

public class MyContext : DbContext {     public override int SaveChanges()     {         ObjectContext context = ((IObjectContextAdapter)this).ObjectContext;          //Find all Entities that are Added/Modified that inherit from my EntityBase         IEnumerable<ObjectStateEntry> objectStateEntries =             from e in context.ObjectStateManager.GetObjectStateEntries(EntityState.Added | EntityState.Modified)             where                 e.IsRelationship == false &&                 e.Entity != null &&                 typeof(EntityBase).IsAssignableFrom(e.Entity.GetType())             select e;          var currentTime = DateTime.Now;          foreach (var entry in objectStateEntries)         {             var entityBase = entry.Entity as EntityBase;              if (entry.State == EntityState.Added)             {                 entityBase.CreatedDate = currentTime;             }              entityBase.LastModifiedDate = currentTime;         }          return base.SaveChanges();     } } 

Answer 2

As i have a service layer mediating between my controllers (im using ASP.NET MVC), and my repository, i have decided to auto-set the fields here.

Also, my POCO's have no relationships/abstractions, they are completely independant. I would like to keep it this way, and not mark any virtual properties, or create base classes.

So i created an interface, IAutoGenerateDateFields:

public interface IAutoGenerateDateFields {    DateTime LastModified { get;set; }    DateTime CreatedOn { get;set; } } 

For any POCO's i wish to auto-generate these fields, i implement this inteface.

Using the example in my question:

public class PocoWithDates : IAutoGenerateDateFields {    public string PocoName { get; set; }    public DateTime CreatedOn { get; set; }    public DateTime LastModified { get; set; } } 

In my service layer, i now check if the concrete object implements the interface:

public void Add(SomePoco poco) {    var autoDateFieldsPoco = poco as IAutoGenerateDateFields; // returns null if it's not.     if (autoDateFieldsPoco != null) // if it implements interface    {       autoDateFieldsPoco.LastModified = DateTime.Now;       autoDateFieldsPoco.CreatedOn = DateTime.Now;    }     // ..go on about other persistence work. } 

I will probably break that code in the Add out to a helper/extension method later on.

But i think this is a decent solution for my scenario, as i dont want to use virtuals on the Save (as i'm using Unit of Work, Repository, and Pure POCO's), and don't want to use triggers.

If you have any thoughts/suggestions, let me know.