M:M table conditional delete for parents

Postgresql 8.1.4 on Redhat 9

I have a table which stores M:M relationships. I can't put foreign keys to
the parents of this table because the relationships being stored go to
several tables. This was done so that only two fields have to be searched
in order for all relationships to be found for an item. For an oem number
there might be 50 to 100 relationships and 40 different tables having to
do with materials, locations, revisions, specifications, customer, etc.
that might be referenced.

Is there some way I can make a mock foreign key restraint on the parents
so the parent would search the M:M table for a matching value in key1 if
the relate-key is 22, 23, 25 or 100 before it allows the row to be
deleted?

relate-key relate-type key1 table1 key2 table2
22 product-material 23 oem 545 material
22 product-material 23 oem 546 material
23 product-engine 23 oem 15 engine
25 product-stage 23 oem 3 stage
100 product-revision 23 oem 2270
specifications

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Margaret Gillon, IS Dept., Chromalloy Los Angeles