ORDER BY value in join table not grouped before aggregation - sql

Question

I am trying to order a Postgres result set based on an array_aggregate function.
I have the following query that works great:
select a.id, a.name, array_agg(f.name)
from actors a
join actor_films af on a.id = actor_id
join films f on film_id = f.id
group by a.id
order by a.id;
This gives me the following results, for example:
id | name | array_agg
----+--------+---------------------------------
1 | bob | {"delta force"}
2 | joe | {"delta force","the funny one"}
3 | fred | {"bad movie",AARRR}
4 | sally | {"the funny one"}
5 | suzzy | {"bad movie","delta force"}
6 | jill | {AARRR}
7 | victor | {"the funny one"}
I want to sort the results so that it is sorted alphabetically by Film name. For example, the final order should be:
id | name | array_agg
----+--------+---------------------------------
3 | fred | {"bad movie",AARRR}
6 | jill | {AARRR}
5 | suzzy | {"bad movie","delta force"}
1 | bob | {"delta force"}
2 | joe | {"delta force","the funny one"}
4 | sally | {"the funny one"}
7 | victor | {"the funny one"}
This is based on the alphabetical name of any movies they are in. When I add the ORDER BY f.name I get the following error:
ERROR: column "f.name" must appear in the GROUP BY clause or be used in an aggregate function
I cannot add it to the group, because I need it aggregated in the array, and I want to sort pre-aggregation, such that I can get the following order. Is this possible?
If you would like reproduce this example, here is the setup code:
create table actors(id serial primary key, name text);
create table films(id serial primary key, name text);
create table actor_films(actor_id int references actors (id), film_id int references film (id));
insert into actors (name) values('bob'), ('joe'), ('fred'), ('sally'), ('suzzy'), ('jill'), ('victor');
insert into films (name) values('AARRR'), ('the funny one'), ('bad movie'), ('delta force');
insert into actor_films(actor_id, film_id) values (2, 2), (7, 2), (4,2), (2, 4), (1, 4), (5, 4), (6, 1), (3, 1), (3, 3), (5, 3);
And the final query with the error:
select a.id, a.name, array_agg(f.name)
from actors a
join actor_films af on a.id = actor_id
join films f on film_id = f.id
group by a.id
order by f.name, a.id;

Answer 1

You can use an aggregation function:
order by min(f.name), a.id