postgres - How to update the same record twice in a joined update query - sql

Question

I'm trying to write the following migration that drops the conversations_users table (which was a join table that included a read_up_to column) and copies the read_up_to information into the new messages.read_by_user_ids array. For every 1 message row there are at least 2 conversations_users rows, so this join is repeating messages. I expected the following expression to work, but it's only assigning one user_id to the read_by_user_ids array, and I'm guessing that's because the update isn't happening sequentially.
Result:
message_id: 1, read_by_user_ids: { 15 }
Desired result: message_id: 1, read_by_user_ids: { 15, 19 }
UPDATE
messages as m
SET
read_by_user_ids = CASE
WHEN cu.read_up_to >= m.created_at THEN array_append(
COALESCE(m.read_by_user_ids, '{}'),
cu.user_id
) ELSE m.read_by_user_ids
END
FROM
conversations_users cu
WHERE
cu.conversation_id = m.thread_id;

Answer 1

I'm on my phone, so apologies for untested typos.
As per my comment, aggregate the individual incoming user_ids into one array per conversation. Then use array_cat to combine the two arrays.
This way you only need to do one update per target row.
I also noticed that you only want to update rows based on a date comparison, so I added that to the sub query I proposed.
UPDATE
messages as m
SET
read_by_user_ids = array_cat(
COALESCE(m.read_by_user_ids, '{}'),
cu.user_id_array
)
FROM
(
SELECT
cu.conversation_id,
array_agg(cu.user_id) AS user_id_array
FROM
messages
INNER JOIN
conversations_users
ON cu.conversation_id = m.thread_id
AND cu.read_up_to >= m.created_at
GROUP BY
cu.conversation_id
)
cu
WHERE
cu.conversation_id = m.thread_id;
There are many other options on how to generate the array in the sub-query. Which is the most efficient will depend on the profile of your data, indexes, etc. But the principle remains the same; updating the same row multiple times in a single statement doesn't work, you need to update each row once, with an array as the input.