I'm pretty sure you have asked this exact question before... And people have said use projections or write your own code to query the content items you need.
So say your content items are notes. And you have attached a NotePart to them. Your NotePart might look like this:
public class NotePartRecord : ContentPartRecord
{
public virtual string Title { get; set; }
public virtual string NoteContent { get; set; }
public virtual UserPartRecord UserPartRecord{ get; set; }
}
The UserPartRecord would be the record of the user who created it. You could then query it like this:
this.services.ContentManager
.Query<NotePart>()
.Where<NotePartRecord>(e => e.UserPartRecord.Id == user.Id)
.List()
where services is IOrchardServices. You could then select the data you want to display or just display the entire content item.
I would recommend looking through Orchards source code, examples of how to do pretty much everything :)