entity framework 5 change log how to implement?

Question

I am creating an application with MVC4 and entity framework 5. How do can I implement this? I have looked around and found that I need to override SaveChanges .

Does anyone have any sample code on this? I am using code first approach.

As an example, the way I am saving data is as follows,

 public class AuditZoneRepository : IAuditZoneRepository
    {
        private AISDbContext context = new AISDbContext();



        public int Save(AuditZone model, ModelStateDictionary modelState)
        {
            if (model.Id == 0)
            {
                context.AuditZones.Add(model);
            }
            else
            {
                var recordToUpdate = context.AuditZones.FirstOrDefault(x => x.Id == model.Id);
                if (recordToUpdate != null)
                {
                    recordToUpdate.Description = model.Description;
                    recordToUpdate.Valid = model.Valid;
                    recordToUpdate.ModifiedDate = DateTime.Now;
                }
            }

            try
            {
                context.SaveChanges();
                return 1;
            }
            catch (Exception ex)
            {
                modelState.AddModelError("", "Database error has occured.  Please try again later");
                return -1;
            }
        }
    }

Answer 1

There is no need to override SaveChanges.

You can

Trigger Context.ChangeTracker.DetectChanges(); // may be necessary depending on your Proxy approach

Then analyze the context BEFORE save. you can then... add the Change Log to the CURRENT Unit of work. So the log gets saved in one COMMIT transaction. Or process it as you see fit. But saving your change log at same time. makes sure it is ONE Transaction.

Analyzing the context sample: I have a simple tool, to Dump context content to debug output so when in debugger I can use immediate window to check content. eg You can use this as a starter to prepare your CHANGE Log. Try it in debugger immediate window. I have FULL dump on my Context class.

Sample Immediate window call. UoW.Context.FullDump();

public void FullDump() {

        Debug.WriteLine("=====Begin of Context Dump=======");
        var dbsetList = this.ChangeTracker.Entries();
        foreach (var dbEntityEntry in dbsetList)
        {

            Debug.WriteLine(dbEntityEntry.Entity.GetType().Name + " => " + dbEntityEntry.State);
            switch (dbEntityEntry.State)
            {
                case EntityState.Detached:
                case EntityState.Unchanged:
                case EntityState.Added:
                case EntityState.Modified:
                    WriteCurrentValues(dbEntityEntry);
                    break;
                case EntityState.Deleted:
                    WriteOriginalValues(dbEntityEntry);
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }
            Debug.WriteLine("==========End of Entity======");
        }
        Debug.WriteLine("==========End of Context======");
    }

    private static void WriteCurrentValues(DbEntityEntry dbEntityEntry)
    {
        foreach (var cv in dbEntityEntry.CurrentValues.PropertyNames)
        {
            Debug.WriteLine(cv + "=" + dbEntityEntry.CurrentValues[cv]);
        }
    }
    private static void WriteOriginalValues(DbEntityEntry dbEntityEntry)
    {
        foreach (var cv in dbEntityEntry.OriginalValues.PropertyNames)
        {
            Debug.WriteLine(cv + "=" + dbEntityEntry.OriginalValues[cv]);
        }
    }
}

EDIT: Get the changes

I use this routine to get chnages...

public class ObjectPair {
    public string Key { get; set; }
    public object Original { get; set; }
    public object Current { get; set; }
 }

public virtual IList<ObjectPair> GetChanges(object poco) {
        var changes = new List<ObjectPair>();
        var thePoco = (TPoco) poco;


        foreach (var propName in Entry(thePoco).CurrentValues.PropertyNames) {
            var curr = Entry(thePoco).CurrentValues[propName];
            var orig = Entry(thePoco).OriginalValues[propName];
            if (curr != null && orig != null) {
                if (curr.Equals(orig)) {
                    continue;
                }
            }
            if (curr == null && orig == null) {
                continue;
            }
            var aChangePair = new ObjectPair {Key = propName, Current = curr, Original = orig};
            changes.Add(aChangePair);
        }
        return changes;
    }

edit 2 If you must use the Internal Object tracking.

    var context =  ???// YOUR DBCONTEXT class
     // get objectcontext from dbcontext... 
     var   objectContext = ((IObjectContextAdapter) context).ObjectContext;
        // for each tracked entry
        foreach (var dbEntityEntry in context.ChangeTracker.Entries()) {
          //get the state entry from the statemanager per changed object
          var stateEntry =  objectContext.ObjectStateManager.GetObjectStateEntry(dbEntityEntry.Entity);
            var modProps = stateEntry.GetModifiedProperties();
            Debug.WriteLine(modProps.ToString());
        }

I decompiled EF6 . Get modified is indeed using private bit array to track fields that have been changed.

 // EF decompiled source..... _modifiedFields is a bitarray
 public override IEnumerable<string> GetModifiedProperties()
{
  this.ValidateState();
  if (EntityState.Modified == this.State && this._modifiedFields != null)
  {
    for (int i = 0; i < this._modifiedFields.Length; ++i)
    {
      if (this._modifiedFields[i])
        yield return this.GetCLayerName(i, this._cacheTypeMetadata);
    }
  }
}