Incrementing a sequence in PostgreSQL based on a foreign key - postgresql

Question

I would like to use sequences to create friendly IDs for some objects in my database. My problem is that I don't want the sequence to be global to the table, instead I want to increment the value based on a foreign key.
For example, my table is defined as:
CREATE TABLE foo (id numeric PRIMARY KEY, friendly_id SERIAL, bar_id numeric NOT NULL)
And I would like friendly_id to increment separately for each bar_id such that the following statements:
INSERT INTO foo (123, DEFAULT, 345)
INSERT INTO foo (124, DEFAULT, 345)
INSERT INTO foo (125, DEFAULT, 346)
INSERT INTO foo (126, DEFAULT, 345)
Would result in (desired behavior):
id | friendly_id | bar_id
-----------+------------------+-----------------
123 | 1 | 345
124 | 2 | 345
125 | 1 | 346
126 | 3 | 345
Instead of (current behavior):
id | friendly_id | bar_id
-----------+------------------+-----------------
123 | 1 | 345
124 | 2 | 345
125 | 3 | 346
126 | 4 | 345
Is this possible using sequences or is there a better way to achieve this?

Answer 1

create table foo (
id serial primary key,
friendly_id integer not null,
bar_id integer not null,
unique(friendly_id, bar_id)
);
At the application wrap the insertion in a exception catching loop to retry if a duplicate key exception is raised
insert into foo (friendly_id, bar_id)
select
coalesce(max(friendly_id), 0) + 1,
346
from foo
where bar_id = 346