Using a query as a lookup table

In my Accounts database, the user sometimes needs to store invoices in a
dated month folder which does not relate to the date on the invoice eg an
Invoice dated 29/12/07 may sometimes have to go in the Jan08 folder. To
allow this, I've put a Year/Month table in my database with a Foreign Key in
the Expenses table so the user can choose the month in which she wishes to
store the invoice but actually, instead of adding items to this Year/Month
table when a new year starts, I could generate the year/months using a query
based on a number table, 1 to 12, combined with a union query which combines
a query containing all the years in the database minus 1 and all the years
in the database plus 1 (so I get the year before the earliest query date and
the year after). To create a 'Primary Key' for this query, I concatenate the
month and year number. All the queries ran fine on their own But, when I
tested it out, by updating a field in my expenses table to hold the correct
'foreign key' for my month/year query and adding my MonthYear query and
creating a query from the 2 tables, the results ran like a total slug. Why
is this? It seems mad to run Append queries to store something as regular as
months.

Evi