Seems like way is prevent the nested prefix on Attachment class is to connect Request & Attachment directly. Still need to observe for a few iteration.
[ActiveRecord]
public class Request : ActiveRecordBase
{
[PrimaryKey]
public int Id {get; set;}
[HasMany(typeof(Attachment)]
public IList Attachments {get; set;}
[Nested("SectionA")]
public SectionA A {get; set;}
[Nested("SectionB")]
public SectionB B {get; set;}
}