gistfile1.txt

[HasMany(typeof(POSPhoto), Inverse = true, Access = PropertyAccess.FieldCamelcaseUnderscore, Cascade = ManyRelationCascadeEnum.All, Lazy = true, RelationType = RelationType.Set )]
public virtual IEnumerable<POSPhoto> Photos
{
	get
	{
		return _photos.AsReadOnly();
	}
}

public virtual void AddPhoto(POSPhoto photo)
{
	photo.Item = this;
	_photos.Add(photo);
}

public virtual void RemoveAllPhotos()
{
    _photos.Clear();
}
item.RemoveAllPhotos();
item.AddPhoto(posPhoto);
item.Save();

Does not delete existing posphotos from db... wth?

So, the issue is that I was setting cascade=all and not cascade=alldeleteorphan.

The important distinction is that alldeleteorphan will remove any objects (not records) that no longer have a parent. In this case, it deletes anything from the db that was removed from the posphoto collection on positem.