Replacing an existing unique constraint (not UNIQUE INDEX) with primary key

Hi all.
 
For historical reasons I have a table which at first had an "id"-column (the
PK) and later got an "entity_id"-column (which is a UNIQUE CONSTRAINT).
 
I'm now trying to get rid of the "id"-column and make the "entity_id"-column
the new PK. The tricky part is that both of these columns are referenced as
FK's from many tables, so disabling/removing FKs is not so easy. I'm facing a
problem when issuing:
ALTER TABLE person ADD PRIMARY KEY USING INDEX person_entity_id_key;
ERROR:  index "person_entity_id_key" is already associated with a constraint

 
A full example of what I'm trying to do (replacing the PK of the
"person"-table) is here:

DROP TABLE IF EXISTS phone; DROP TABLE IF EXISTS address; DROP TABLE IF EXISTS
person;CREATE TABLE person ( id BIGINT PRIMARY KEY, entity_id BIGINT NOT NULL
UNIQUE, name VARCHAR NOT NULL ); CREATE TABLE address ( id BIGINT PRIMARY KEY,
person_idBIGINT NOT NULL REFERENCES person (id) ); CREATE TABLE phone ( id
BIGINT PRIMARY KEY, person_entity_id BIGINT NOT NULL REFERENCES person
(entity_id), numberVARCHAR NOT NULL ); INSERT INTO person (id, entity_id, name)
VALUES(1, 101, 'Andreas'), (2, 102, 'Santa'); INSERT INTO address (id,
person_id)VALUES (1, 1), (2, 2); INSERT INTO phone (id, person_entity_id,
number)VALUES (1, 101, '1800555123'), (2, 102, '1800555456'); -- Drop the
deprecated foreign key on addressALTER TABLE address DROP CONSTRAINT
address_person_id_fkey;-- Update address and make person_id point to
person.entity_id instead of person.idUPDATE address a SET person_id =
p.entity_idFROM person p WHERE p.id = a.person_id; ALTER TABLE address ADD
FOREIGN KEY(person_id) REFERENCES person (entity_id); -- Drop the deprecated
id-columnALTER TABLE person DROP COLUMN id; -- Try to make new PK using the
UNIQUE CONSTRAINT person_entity_id_keyALTER TABLE person ADD PRIMARY KEY USING
INDEXperson_entity_id_key; ERROR:  index "person_entity_id_key" is already
associated with a constraint
 
 
I see that if I had declared person.entity_id without the UNIQUE-keyword and
instead created a UNIQUE INDEX:
create UNIQUE INDEX person_entity_id_key on person(entity_id);  
Then the ADD PRIMARY KEY USING INDEX command would have succeeded.
 
I have lots of queries which have GROUP BY person.id which now should use
GROUP BY person.entity_id, and not having to also list all other columns
selected from the person-table.
 
How do I proceed with this?

 
Thanks.
 
-- Andreas Joseph Krogh
CTO / Partner - Visena AS
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