mysql - Foreign key pointing to different tables -

i'm implementing table per subclass design discussed in previous question. it's product database products can have different attributes depending on type, attributes fixed each type , types not manageable @ all. have master table holds common attributes:

product_type ============  product_type_id int product_type_name varchar  e.g.:  1 'magazine' 2 'web site'  product =======  product_id int product_name varchar product_type_id int -> foreign key product_type.product_type_id valid_since datetime valid_to datetime  e.g.  1 'foo magazine'      1 '1998-12-01' null 2 'bar weekly review' 1 '2005-01-01' null 3 'e-commerce app'    2 '2009-10-15' null 4 'cms'               2 '2010-02-01' null 

... , 1 subtable each product type:

item_magazine =============  item_magazine_id int title varchar product_id int -> foreign key product.product_id issue_number int pages int copies int close_date datetime release_date datetime  e.g.  1 'foo magazine regular issue'      1 89 52 150000 '2010-06-25' '2010-06-31' 2 'foo magazine summer special'     1 90 60 175000 '2010-07-25' '2010-07-31' 3 'bar weekly review regular issue' 2 12 16  20000 '2010-06-01' '2010-06-02'  item_web_site =============  item_web_site_id int name varchar product_id int -> foreign key product.product_id bandwidth int hits int date_from datetime date_to datetime  e.g.  1 'the carpet store'        3 10  90000 '2010-06-01'         null 2 'penauts r us'            3 20 180000 '2010-08-01'         null 3 'springfield cattle fair' 4 15 150000 '2010-05-01' '2010-10-31' 

now want add fees relate 1 specific item. since there little subtypes, it's feasible this:

fee ===  fee_id int fee_description varchar item_magazine_id int -> foreign key item_magazine.item_magazine_id item_web_site_id int -> foreign key item_web_site.item_web_site_id net_price decimal  e.g.:  1 'front cover'      2 null 1999.99 2 'half page'        2 null  500.00 3 'square banner' null    3  790.50 4 'animation'     null    3 2000.00 

i have tight foreign keys handle cascaded editions , presume can add constraint 1 of ids not null.

however, intuition suggests cleaner rid of item_whatever_id columns , keep separate table:

fee_to_item ===========  fee_id int -> foreign key fee.fee_id product_id int -> foreign key product.product_id item_id int -> ??? 

but can't figure out how create foreign keys on item_id since source table varies depending on product_id. should stick original idea?

update

the alternative considering is:

fee ===  fee_id int fee_description varchar product_id int -> foreign key product.product_id item_id int -> ??? net_price decimal 

i'm not sure why mentioned separate fee_to_item table (i guess thinking of else) doesn't change question since key point same: foo1_id+foo2_id+foo3_id vs source_id+foo_id

i go 1 fk column , flag column. instead of

fee  ===   fee_id int  fee_description varchar  item_magazine_id int -> foreign key item_magazine.item_magazine_id  item_web_site_id int -> foreign key item_web_site.item_web_site_id  net_price decimal  

you'd have

fee  ===   fee_id int  fee_description varchar  item_id int -> foreign key item_magazine/website.item_magazine/website_id  product_type_id int -> foreign key product_type.product_type_id net_price decimal  

then queries generalizable. instead of 2 different queries, e.g.:

select * fee item_magazine_id=x select * fee item_website_id=y 

you like:

select * fee item_id=x , product_type_id=1 select * fee item_id=y , product_type_id=2