A question about the number of times a trigger will fire

I am in the process of trying to set up a function.trigger pair to create
functionally an updateable view.

I would think that the trigger would fire the number of times that the
calling statement's WHERE clause seceded. Is this incorrect>

I have a view called purchase_view, one of the tables in it's join is a
table called bom_item.

I have defined this trigger:

REATE TRIGGER test_v_trig
INSTEAD OF INSERT OR UPDATE ON purchase_view
FOR EACH ROW EXECUTE PROCEDURE v_trig_test();

Prior to firing this trigger this query

select count(*)
FROM purchase_view
WHERE
proj_no = 3124
AND
m_name = 'Mencom' ;

returns 11

The resultant statement generated by the function called by this trigger
is:

UPDATE BOM_ITEM SET cost_per_unit = 23.45 , qty = 12345.00

Which in retrospect dies not do what I had in mind :-)

So, it appears that I need to create a WHERE clause for the resultant
statement. But I do not see how the function has enough data to use to
create this where clause.

What am I missing, here?

--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin