Compound foreign key constraint with a String literal - postgresql

Question

I have a table with a compound primary key:
Table "account.enum"
Column | Type | Modifiers
-----------+------------------------+-----------
classname | character varying(256) | not null
name | character varying(64) | not null
active | boolean | not null
Indexes: "enum_pkey" PRIMARY KEY, btree (classname, name)
Values:
classname | name | active
--------------+--------+--------
CURRENCY | EUR | t
CURRENCY | USD | t
MUTATIONTYPE | CREDIT | t
MUTATIONTYPE | DEBET | t
Another table account uses this table:
Table "account.mutation"
Column | Type | Modifiers
-----------------+------------------------+-------------------------------------------------------
id | bigint | not null default nextval('mutation_id_seq'::regclass)
accountnumber | character varying(9) | not null
interestdate | date | not null
balancebefore | numeric(10,2) | not null
balanceafter | numeric(10,2) | not null
transactiondate | date | not null
amount | numeric(10,2) | not null
description | character varying(512) | not null
ordernumber | smallint | default (-1)
mutationtype | character varying(64) |
currency | character varying(64) |
I want to add foreign key constraints (for mutationtype and currency):
alter table mutation add constraint FK_mutationtype foreign key('MUTATIONTYPE', mutationtype) references enum(classname, name);
alter table mutation add constraint FK_currency foreign key('CURRENCY', currency) references enum(classname, name);
However the string literals are not accepted.
What am I doing wrong? Is what I want possible in postgres?

Answer 1

You cannot do this because foreign keys can only be defined on columns, not on expressions like the literal 'MUTATIONTYPE'.
You could introduce a column like mutation_class in mutation that is always set to 'MUTATIONTYPE', but that sounds wasteful, redundant and silly.
I think that you should solve the problem by having different lookup tables for the different enumerations, then the difficulty would just vanish, and the whole design looks more reasonable from a relational perspective.