Perform UNNEST, INNER JOIN and then ARRAY_AGG as part of an UPDATE query

I am trying to unnest an array from one table using ORDINALITY to preserve order, then perform an INNER JOIN on another table to find the corresponding value from a specific column and then use ARRAY_AGG to package this back up and UPDATE the original table. I have something working for a single query, but I want to do an UPDATE on each row in a table but can’t seem to get it to work. I feel like I’m close but I’m spending too long on this so any help would be appreciated.

Code is below to generate the tables, alongside the answer I am looking for and my attempts.

create table table_1(
    table_1_id int,
    table_2_id_list int[],
    table_2_geom text[]
);

insert into table_1 values 
    (1, ARRAY[1,3,5], null) ,(2, ARRAY[2,4,6], null);

create table table_2(table_2_id int, geom text);
insert into table_2 values
    (1, 'geom1'), (2, 'geom2'), (3, 'geom3'),
    (4, 'geom4'), (5, 'geom5'), (6, 'geom6');

I want to end up with this:

table_1_id  | table_2_id_list  | table_2_geom
------------------------------------------------------------------
1           | (1, 3, 5)        |  (geom1, geom3, geom5) 
2           | (2, 4, 6)        |  (geom2, geom4, geom6) 

I can get it working for the single case using the following:

SELECT 
    TABLE_1_ID, 
    array_agg(TABLE_2.geom ORDER BY ORDINALITY) 
FROM TABLE_1, 
unnest(table_2_id_list) WITH ORDINALITY a 
INNER JOIN TABLE_2 ON a = TABLE_2.TABLE_2_ID 
GROUP BY TABLE_1_ID LIMIT 1;

But when I try to do something similar to UPDATE each row in the table, I do something wrong. I’ve tried the following but it doesn’t work:

UPDATE TABLE_1
SET table_2_geom = (
    SELECT array_agg(TABLE_2.geom ORDER BY ORDINALITY) 
    FROM TABLE_1, 
    unnest(table_2_id_list) WITH ORDINALITY a 
    INNER JOIN TABLE_2 ON a = TABLE_2.TABLE_2_ID
); 

If anyone can point me in the correct direction I would be very grateful.

Thanks

Answer

You could turn your existing query to a CTE and join it back with the original table for update:

with cte as (
    select 
        t1.table_1_id, 
        array_agg(t2.geom order by ordinality) table_2_geom
    from 
        table_1 t1
        cross join lateral unnest(t1.table_2_id_list) with ordinality i(table_2_id) 
        inner join table_2 t2 on t2.table_2_id = i.table_2_id
    group by t1.table_1_id
)
update table_1 t1
set table_2_geom = c.table_2_geom
from cte c
where c.table_1_id = t1.table_1_id

Demo on DB Fiddle – table content after the update:

table_1_id | table_2_id_list | table_2_geom       
---------: | :-------------- | :------------------
         1 | {1,3,5}         | {geom1,geom3,geom5}
         2 | {2,4,6}         | {geom2,geom4,geom6}

But a correlated subquery might be simpler:

update table_1 t1
set table_2_geom = (
    select array_agg(t2.geom order by ordinality)
    from unnest(t1.table_2_id_list) with ordinality i(table_2_id)
    inner join table_2 t2 on t2.table_2_id = i.table_2_id

)

Demo on DB Fiddle