I'm trying to insert records using the esDataSource and a DetailsView which is nested within a Gridview.

The DetailsView only shows a single textbox and a commandbutton (insert). I've trapped the esDataSource_esPreInsert() event to add the rest of the values like so (values contain good values, just changed for readability):

Private Sub esDataSource1_esPreInsert(ByVal sender As Object, ByVal e As EntitySpaces.Web.esDataSourceInsertEventArgs) Handles esDataSource1.esPreInsert
        
        e.Values("SiNumber") = value1
        e.Values("PnmMain") = value2
        e.Values("PnmAutoKey") = value3
        e.Values("InspectionType") = value4
        e.Values("Description") = value5

End Sub

You should only have to implement esCreateEntity

protected void EsDataSource1_esCreateEntity(object sender, EntitySpaces.Web.esDataSourceCreateEntityEventArgs e)
{
	Employees entity = new Employees();

	if (e.PrimaryKeys != null)
	{
		entity.LoadByPrimaryKey((int)e.PrimaryKeys[0]);
	}

	// Assign the Entity
	e.Entity = entity;
}

Yes I had already done this.

The problem is that my DetailsView should only have one field (partnumber). The other fields should not be entered by the user, these are retrieved by the system. These fields cannot be Null when inserting, so I need to fill them manually before the insert is performed... any suggestions?

Yes, on your esDataSourceInsertEventArgs in your PreInsert method you will find your entity, go ahead and set fields:

e.Entity.SomeField = "data";

See the documentation on the esDataSourceInsertEventArg class. The esDataSource always hands you your entity on the events.