Okay, I see some other people have asked questions about this error:
ORA-01779: cannot modify a column which maps to a non key-preserved table

I still can't figure it out. I would appreciate any help you can provide. Here is my scenario: I am trying to update multiple fields on specific records in a table based on other rows in the same table.
I am sure that there is a one-to one match in my join, but I still get that error.

CREATE TABLE MY_RATIOS
(
  SEQ_NO                          NUMBER(9)     NOT NULL,
  SCENARIO                        VARCHAR2(250 BYTE) NOT NULL,
  REC_TYP                         VARCHAR2(2 BYTE) NOT NULL,
  RATIO_1                         FLOAT(126),
  RATIO_2                         FLOAT(126));
alter table MY_RATIOS add constraint t2_pk primary key(SEQ_NO, SCENARIO, REC_TYP);

Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (1, 'current year', 'AR', 25, 75);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (1, 'forecast', 'AR', 0, 0);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (2, 'current year', 'AR', 10, 90);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (2, 'forecast', 'AR', 0, 0);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (3, 'current year', 'AR', 92, 8);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (3, 'forecast', 'AR', 46, 54);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (4, 'current year', 'AR', 18, 82);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (5, 'forecast', 'AR', 51, 49);
Insert into MY_RATIOS (SEQ_NO, SCENARIO, REC_TYP, RATIO_1, RATIO_2) Values (3, 'current year', 'AX', 28, 72);

So, where there are records where scenario='forecast' and rec_typ='AR' and the sum of the ratios=0, I want to update the ratios with the ratios from the corresponding records from the same table that have the same seq_no, rec_typ and scenario='current year' without impacting any of the other records.

Here is my attempt at the sql:

update ( 
select a.seq_no, a.RATIO_1 as new_RATIO_1, a.RATIO_2 as new_RATIO_2, b.RATIO_1, b.RATIO_2 from my_ratios a, my_ratios b
where a.SEQ_NO = b.SEQ_NO 
and a.scenario = 'forecast' and b.scenario = 'current year'
and a.rec_typ = b.rec_typ
and a.rec_typ = 'AR'
and a.RATIO_1 + a.RATIO_2 = 0
)
set new_RATIO_1 = RATIO_1, new_RATIO_2 = RATIO_2;

this results in:
ORA-01779: cannot modify a column which maps to a non key-preserved table

Here are a couple of things I tried which might help:
1) If I just run the select statement from the sql above, I get the expected results.

select a.seq_no, a.RATIO_1 as new_RATIO_1, a.RATIO_2 as new_RATIO_2, b.RATIO_1, b.RATIO_2 from my_ratios a, my_ratios b
where a.SEQ_NO = b.SEQ_NO 
and a.scenario = 'forecast' and b.scenario = 'current year'
and a.rec_typ = b.rec_typ
and a.rec_typ = 'AR'
and a.RATIO_1 + a.RATIO_2 = 0

Results:
seq_no new_ratio_1 new_ratio_2 ratio_1 ratio_2
1 0 0 25 75
2 0 0 10 90

2) If I change this line in the where clause:
and a.scenario = 'forecast' and b.scenario = 'current year'
to
and a.scenario = b.scenario

I don't get the error, but doesn't do what I need it to do.

Please keep in mind that I am not a DBA and do not have access to modify any tables or indexes, so a solution that does not require that is preferred.

Thanks Tom!

ops$tkyte%ORA11GR2> merge into
  2  (select *
  3     from my_ratios
  4    where scenario = 'forecast'
  5      and rec_typ = 'AR'
  6      and ratio_1+ratio_2 = 0
  7  ) a
  8  using
  9  (select *
 10     from my_ratios
 11    where scenario = 'current year'
 12      and rec_typ = 'AR'
 13  ) b
 14  on (a.seq_no = b.seq_no)
 15  when matched then update set a.ratio_1 = b.ratio_1, a.ratio_2 = b.ratio_2;

2 rows merged.