PostgreSQL migration from id(bigint) to id(uuid) while preserving relations - ruby-on-rails

Question

I have many tables and a lot of data and I would like to change all primary key types from bigint to uuid type. I have no idea how to preserve relations. Here is an example.
This means I am planning to
Add new column uuid of type uuid
Rename id column to id_obsolete
Rename uuid to id
Now I somehow need to preserve relations, so for example, if I have a table
users that has person_id (bigint) column, this should get migrated to person_id (uuid) and maintain the relationship. How can I do this without losing any data?

Answer 1

First, let's create sample data:
create table employees
(
id int4,
emp_name varchar,
emp_surname varchar
);
insert into employees (id, emp_name, emp_surname) values (100, 'Tim', 'James');
insert into employees (id, emp_name, emp_surname) values (101, 'Bella', 'Tucker');
insert into employees (id, emp_name, emp_surname) values (102, 'Ryan', 'Metcalfe');
insert into employees (id, emp_name, emp_surname) values (103, 'Dominic', 'King');
create table transactions
(
id int4,
emp_id int4,
tran_date date,
total int4
);
insert into transactions (id, emp_id, tran_date, total) values (1, 100, now(), 120);
insert into transactions (id, emp_id, tran_date, total) values (2, 100, now(), 195);
insert into transactions (id, emp_id, tran_date, total) values (3, 100, now(), 250);
insert into transactions (id, emp_id, tran_date, total) values (4, 100, now(), 50);
insert into transactions (id, emp_id, tran_date, total) values (5, 101, now(), 70);
insert into transactions (id, emp_id, tran_date, total) values (6, 101, now(), 125);
insert into transactions (id, emp_id, tran_date, total) values (7, 102, now(), 600);
insert into transactions (id, emp_id, tran_date, total) values (8, 102, now(), 15);
insert into transactions (id, emp_id, tran_date, total) values (9, 102, now(), 90);
insert into transactions (id, emp_id, tran_date, total) values (10, 103, now(), 10);
insert into transactions (id, emp_id, tran_date, total) values (11, 103, now(), 60);
insert into transactions (id, emp_id, tran_date, total) values (12, 103, now(), 155);
insert into transactions (id, emp_id, tran_date, total) values (13, 103, now(), 30);
create table item_sales
(
id int4,
emp_id int4,
process_date date,
price int4
);
insert into item_sales (id, emp_id, process_date, price) values (1, 100, now(), 5);
insert into item_sales (id, emp_id, process_date, price) values (2, 101, now(), 7);
insert into item_sales (id, emp_id, process_date, price) values (3, 102, now(), 12);
insert into item_sales (id, emp_id, process_date, price) values (4, 101, now(), 5);
insert into item_sales (id, emp_id, process_date, price) values (5, 103, now(), 9);
insert into item_sales (id, emp_id, process_date, price) values (6, 102, now(), 12);
insert into item_sales (id, emp_id, process_date, price) values (7, 100, now(), 9);
insert into item_sales (id, emp_id, process_date, price) values (8, 101, now(), 5);
insert into item_sales (id, emp_id, process_date, price) values (9, 100, now(), 9);
insert into item_sales (id, emp_id, process_date, price) values (10, 103, now(), 1);
insert into item_sales (id, emp_id, process_date, price) values (11, 102, now(), 6);
We need convert id field on employees to uuid and update all emp_id on another tables. First, create new uuid field.
ALTER TABLE employees
ADD id_new uuid;
ALTER TABLE item_sales
ADD emp_id_new uuid;
ALTER TABLE transactions
ADD emp_id_new uuid;
Update our new uuid field using generate uuid function:
update employees
set id_new = uuid_generate_v4();
Then update another tables using join:
update item_sales sls
set emp_id_new = emp.id_new
from employees emp
where emp.id = sls.emp_id;
update transactions trn
set emp_id_new = emp.id_new
from employees emp
where emp.id = trn.emp_id;
After then we can delete old id and emp_id fields and rename new fields to old name.
ALTER TABLE employees
DROP COLUMN id;
ALTER TABLE item_sales
DROP COLUMN emp_id;
ALTER TABLE transactions
DROP COLUMN emp_id;
ALTER TABLE employees
RENAME COLUMN id_new TO id;
ALTER TABLE item_sales
RENAME COLUMN emp_id_new TO emp_id;
ALTER TABLE transactions
RENAME COLUMN emp_id_new TO emp_id;