Howto implement Lazy load with DAAB

NickyLarson posted on Saturday, September 12, 2009

Hello everyone.

I don´t want to use Linq or EF yet.

The projectTracker example

  private void DataPortal_Fetch(SingleCriteria<Project, Guid> criteria)
    {
      using (var ctx = ContextManager<ProjectTracker.DalLinq.PTrackerDataContext>.GetManager(ProjectTracker.DalLinq.Database.PTracker))
      {
        // get project data
        var data = (from p in ctx.DataContext.Projects
                    where p.Id == criteria.Value
                    select p).Single();

        LoadProperty(IdProperty, data.Id);
        LoadProperty(NameProperty, data.Name);
        LoadPropertyConvert<SmartDate, System.DateTime?>(StartedProperty, data.Started);
        LoadPropertyConvert<SmartDate, System.DateTime?>(EndedProperty, data.Ended);
        LoadProperty(DescriptionProperty, data.Description);
        _timestamp = data.LastChanged.ToArray();

        // get child data
        LoadProperty(
        ResourcesProperty,
        ProjectResources.GetProjectResources(data.Assignments.ToArray())); <--- Linq Entity Set
      }
    }

How can I  do the same thing but with ADO.Net (DAAB)

Thanks

Nicky

JonnyBee replied on Saturday, September 12, 2009

Hi,

When using pure ADO.NET  you will have use the DataReader /SafeDataReader to get your data.

Alt 1: Use stored procedures that return multiple recordsets and pass the DataReader as parameter.
Alt 2: Use direct sql/command to get to each table and read data from the datareader.

Alt1 is shown in ProjectTracker sample for Csla 3.0.5.

/jonnybee

NickyLarson replied on Saturday, September 12, 2009

Hi jonnybee

Thanks for the answer..

I will check this example ProjectTracker for Csla 3.0.5.

Nicky

JonnyBee replied on Saturday, September 12, 2009

Hi Nick,

Your question was about "Lazy loading" - but the code sample you provided does not show LazyLoading .

LazyLoading is when you do not fetch the child list upon initial fetch of the root - but does a fetch of values when the property is accessed.

Conceptually the lazy loaded list is a:

• EditableChildList/ReadOnlyChildList with a DataPortal.Fetch method.
  
• Updates on editable child list is executed as part og the Save operation on the root object.

/jonnybee

NickyLarson replied on Sunday, September 13, 2009

Hi jonnybee

The code I post, in the book of Rocky he says
"It is important to realize that the data.Assignments.ToArray() call causes LINQ to SQL to make
a second query to the database to retrieve the list of assignments. LINQ to SQL uses a lazy loading
scheme and only retrieves data when necessary."

That´s why I talk about lazy load.

About the example you tell me to  see in ProjectTracker sample for Csla 3.0.5.

   private void DataPortal_Fetch(Criteria criteria)
    {
      using (SqlConnection cn = new SqlConnection(Database.PTrackerConnection))
      {
        cn.Open();
        using (SqlCommand cm = cn.CreateCommand())
        {
          cm.CommandType = CommandType.StoredProcedure;
          cm.CommandText = "getProject";
          cm.Parameters.AddWithValue("@id", criteria.Id);

          using (SafeDataReader dr = new SafeDataReader(cm.ExecuteReader()))
          {
            dr.Read();
            _id = dr.GetGuid("Id");
            _name = dr.GetString("Name");
            _started = dr.GetSmartDate("Started", _started.EmptyIsMin);
            _ended = dr.GetSmartDate("Ended", _ended.EmptyIsMin);
            _description = dr.GetString("Description");
            dr.GetBytes("LastChanged", 0, _timestamp, 0, 8);

            // load child objects
            dr.NextResult();
            _resources.ListChanged -= new System.ComponentModel.ListChangedEventHandler(_resources_ListChanged); <--- Do I need this ? I use CSLA 3.7.0
            _resources = ProjectResources.GetProjectResources(dr);
            _resources.ListChanged += new System.ComponentModel.ListChangedEventHandler(_resources_ListChanged); <--- Do I need this ? I use CSLA 3.7.0
          }
        }
      }
    }


One more question if I may. Howto to implement using pure ADO.NET lazy load child?
I have to do all the work, I have to call the MarkAsChild method ?

Thanks

Nicky

rfcdejong replied on Sunday, September 13, 2009

data.Assignments.ToArray() does make the call for lazy loading in L2SQL, but u do it in the fetch method so it becomes a eager fetch for the business object the way u do it. Even with L2S u should do data.Assignments.ToArray() in a property using the following code (for sync lazy loading)

public static PropertyInfo resourcesProperty = .. register theproperty
public ResourcesList Resources
{
get
{
if (!FieldManager.FieldExists(fieldname))
{
int resourceId = ReadProperty(ResourceIdProperty);
LoadProperty(resourcesProperty, ProjectResources.GetProjectResource(resourceId));
}
return GetProperty(resourcesProperty);
}
set
{
...
}
}

ofcourse there only a reason to do lazy loading if u know that the UI won't access the property often

NickyLarson replied on Monday, September 14, 2009

Hi rfcdejong

Yes you are right..

Thanks for open my eyes.

Nicky