Question

I have, e.g. a ‘CreateDate’ property on all my entity classes, using EF 4.3 Code First. I would like to prevent any changes to this field ever being send to the database, except for when the object is first created.

Now I could make this property read only, but I am using an MVC4 front end, and this requires a new object to be instantiated and bound to incoming property values during HTTP POST operations. I could then read the original object and set values based on that, which is not too efficient, or store the value in a hidden field, which is not too secure.

I would preferably like to simply suppress any update to this field when my DbContext saves changes. Can I do this? How?

Answer 1

I would use a marker interface for the entities which have CreateDate then I would override the DbContext.SaveChanges and then with the help of the ChangeTracker set or reset the CreatedAt value.

Maybe it sounds complicated but I’ve created a little sample where you can get started:

public interface ICreatedAt
{
    DateTime CreatedAt { get; set; }
}

public class MyEntity : ICreatedAt
{
    public int Id { get; set; }

    public DateTime CreatedAt { get; set; }
}

public class MyContext : DbContext
{
    public DbSet<MyEntity> MyEntities { get; set; }

    public override int SaveChanges()
    {
        foreach (var entry in ChangeTracker.Entries<ICreatedAt>())
        {
            if (entry.State == System.Data.EntityState.Added)
            {
                entry.Entity.CreatedAt = DateTime.Now;
            }
            if (entry.State == System.Data.EntityState.Modified)
            {
                entry.Entity.CreatedAt = 
                    entry.OriginalValues.GetValue<DateTime>("CreatedAt");
            }
        }
        return base.SaveChanges();
    }
}