M:M table conditional delete for parents

[MargaretGillon@xxxxxxxxxxxxxx]

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