How to enforce subtypes/supertypes in Access 2000?

Hi folks,

I am creating a client database in MS Access with the following (simplified)
table structu

tblContacts:
ContactID (pk - autonumber)
ContactType (fk) (from tblContactTypes)
ContactDetails (text)

tblIndividuals
IndivID (pk - autonumber)
ContactID (fk) (from tblContacts)
IndivDetails (txt)

tblOrganisations
OrgID (pk - autonumber)
ContactID (fk) (from tblContacts)
OrgDetails (txt)

tblContactTypes (serves as a lookup table)
tblContactTypeID (pk - autonumber)
tblContactType (txt - contains values "Indiv" or "Org")

There is a 1:1 relationship between the ContactID (pk) in tblContacts and
the ContactID (fks) in tblIndividuals and tbleOrganisations.

I have created a form in MS Access for entering client details. At the
moment, I have two subforms - frmIndiv and frmOrg - which are positioned on
my main entry form.

The form contains a combo-box from which the user can select "Indiv" or
"Org" as the ContactType. Depending on the value in the combo-box, one or
other of the two subforms will become visible.

At the moment, the user selects - say - "Indiv" as the ContactType and
proceeds to enter details for this type of Contact. When this happens, the
ContactID for the current record in tblContacts table is mirrored in the
ContactID foreign key in the tblIndividuals table. This is what I want.

The problem is that once the user is finished (and whilst still in the same
record in the tblContacts table), the user can select "Org" from the
combo-box and be provided with a empty copy of the sub-form frmOrg. If the
user proceeds to enter data on the sub-form, the ContactID foreign key in
the frmOrg will also mirror the ContactID in tblContacts.

This results in a record in both of my subtype tables (tblIndividuals and
tblOrganisations) having a record which points to the same ContactID in the
supertype table (tblContacts).

How can I prevent this from happening? - ie make sure that each record in
the subtype tables points to a record in the supertype table for which no
subtype record has already been created? (That's a mouthful - I hope it
makes sense). I've seen some references to "check constraints" on the
internet which I believe might help achieve my objective. But - so far as I
am aware - I can't impose check constraints on fields in Access 2000. (I
have seen a suggestion that this might be achieved by using ADO, but no code
example was given).

Any pointers would be appreciated.

Please note, I am a complete novice at this.


TIA
Bob