Conditional Unique Constraint, Ignored for Same Foreign Key - sql

Question

I have a situation where I need to maintain a unique constraint for two columns (FK_1, NAME) conditionally. The constraint should not apply for rows where a second foreign key is the same (FK_2).
ID FK_1 FK_2 NAME
1 2 3 'X01'
2 2 3 'X01-A'
3 2 3 'X01' --Accepted
4 2 4 'X01' --Violation
5 3 5 'X01' --Accepted
What I'm trying to accomplish is that rows 1, 2, and 3 are valid, because FK_2 is the same. They break the unique constraint, but the unique constraint is ignored because FK_2 is the same. Row 4 will violate the unique constraint for (FK_1,NAME) because FK_2 would have been new. And finally, row 5 is fine due to the normal function of a unique constraint on (FK_1,NAME).
It's trying to determine how to handle row 3 that is the complication.
I've seen several examples of how to address this same problem when the equivalent of FK_2 is hard-coded to some extent (Oracle: function based index selective uniqueness), but I'm not sure how I can address this for a foreign key, where I really don't have control over the value of FK_2.
I have tried implementing a function-based unique index, but it results in a ORA-04091 exception.
CREATE OR REPLACE FUNCTION UNQ_TEST_FUNCTION(var_fk_1 IN NUMBER, var_fk_2 IN NUMBER, var_name IN VARCHAR2) RETURN NUMBER DETERMINISTIC IS
result NUMBER;
BEGIN
result := null;
IF (var_fk_1 IS NULL OR var_fk_2 IS NULL OR var_name IS NULL ) THEN
result := null;
ELSE
SELECT COUNT(ID) INTO result FROM TEST1 WHERE
FK_2 != var_fk_2 AND FK_1 = var_fk_1 AND NAME = var_name;
IF (result <= 0) THEN
result := null;
ELSE
result := 1;
END IF;
END IF;
RETURN result;
END;
CREATE UNIQUE INDEX UNQ_TEST ON TEST1
(
UNQ_TEST_FUNCTION("FK_1","FK_2","NAME")
)
;
INSERT INTO TEST1 (ID, FK_1, FK_2, NAME)
VALUES (1, 2, 3, 'X01');
ORA-04091: table TEST1 is mutating, trigger/function may not see it
The exception makes perfect sense, but being unable to query the table puts me at a loss about where to go.

Answer 1

An interesting problem. I would approach this in the following way:
CREATE TABLE t123_fk(
FK_1 int,
FK_2 int,
CONSTRAINT t123_fk_pk PRIMARY KEY(FK_1, FK_2),
CONSTRAINT fk_2_is_new_constr_violated UNIQUE(FK_1)
);
CREATE TABLE t123(
ID int,
FK_1 int,
FK_2 int,
NAME varchar2(100),
constraint t123_fk FOREIGN KEY(FK_1, FK_2) REFERENCES t123_fk
);
CREATE OR REPLACE TRIGGER some_name
BEFORE INSERT OR UPDATE ON t123
FOR EACH ROW
BEGIN
INSERT INTO t123_fk( fk_1, fk_2 )
SELECT :new.FK_1, :new.FK_2 FROM dual
WHERE NOT EXISTS(
SELECT 1 FROM t123_fk
WHERE fk_1 = :new.fk_1 AND fk_2 = :new.fk_2
);
END;
/
Table t123 is the main table containing our data.
Table t123_fk and the trigger are auxiliary and they are used only to help us to force our constraint (btw, our constraint is named fk_2_is_new_constr_violated in the above code).
Here is a test - the fourth insert will be rejected by the database:
insert into t123( id, fk_1, fk_2, name) values(1,2,3,'X01');
insert into t123( id, fk_1, fk_2, name) values(2,2,3,'X01-A');
insert into t123( id, fk_1, fk_2, name) values(3,2,3,'X01');
insert into t123( id, fk_1, fk_2, name) values(4,2,4,'X01'); // this insert will be rejected
insert into t123( id, fk_1, fk_2, name) values(5,3,5,'X01');