Circular Dependencies From Constraints

Hey!

 I am creating a kind of file browser for an application of mine. The principle is quite straight forward. It consists of folders and files. Each folder can contain other folders and files and so on. The twitch however is that i need a special root entity called site. The site is very much alike a folder but has some other properties. The site can contain folders and files.

To achieve this ive created te following tables (truncated for clearity):

###################
#            Sites          #
###################
# ID [Int]                   #
# ...                         #
###################

###################
#          Folders         #
###################
# ID [Int]                   #
# SiteID [Int]             #
# FolderID [Int]          #
# ...                         #
###################

###################
#          Files             #
###################
# ID [Int]                   #
# SiteID [Int]             #
# FolderID [Int]          #
# ...                         #
###################

 Both the folder table and the files table have a check constraint that ensures that either SiteID or FolderID is NULL. They WILL be part of EITHER a folder or a site. Not both!

Then i set up the foreign constraints as follows:
Folders.FolderD -> Folders.ID
Folders.SiteID -> Sites.ID
Files.FolderID -> Folders.ID
Files.SiteID -> Sites.ID

All constraints have cascade on delete and therefore the last of them cannot be created as it would be circular. (Wich it wont in this case, but theoreticaly its possible)

Iknow WHY this is rendering an error. But how can i work around it? Or would you suggest another design of the tables?