Ecto Query: Count/1 of left_joined value multiplied by outer_join value count - sql

Question

I have an items postgres table that has_many bookmarks and has_many notes. I am querying these with {:ecto_sql, "~> 3.7"}.
I want a query that returns all finished items that have notes, and I want that query to also count that item's bookmarks.
When I left_join a note's bookmarks and select_merge count(bookmarks), I get the proper count, but when I add an inner_join notes to the item, the count of bookmarks is multiplied by the number of notes, e.g. if an item has 2 bookmarks and 4 notes, bookmark_count will be 8 when it should be 2.
Here is my funky ecto query:
from item in Item,
where: item.finished == true,
left_join: bookmark in assoc(item, :bookmarks),
on: bookmark.item_id == item.id and bookmark.deleted == false,
select_merge: %{bookmark_count: count(bookmark)},
inner_join: note in assoc(item, :notes),
on: note.accepted == true
Many thanks in advance for feedback/guidance!

Answer 1

Basically: aggregate the N-side before joining to avoid multiplying rows from the main table. Faster, too. See:
Two SQL LEFT JOINS produce incorrect result
Use a semi-join for notes with EXISTS to only verify the existence of a related qualifying row. This also never multiplies rows.
This query should implement your objective:
SELECT i.*, COALESCE(b.ct, 0) AS bookmark_count
FROM items i
LEFT JOIN (
SELECT b.item_id AS id, count(*) AS ct
FROM bookmarks b
WHERE NOT b.deleted
GROUP BY 1
) b USING (id)
WHERE i.finished
AND EXISTS (
SELECT FROM notes n
WHERE n.item_id = i.id
AND n.accepted
);
I slipped in a couple other minor improvements.