Limit returned entries per user per query - node.js

Question

I have the following request:
let artPerCall = 20
let artPerUser = 2
let start = req.params.page
let query = `
SELECT * from
(
SELECT a.*, row_to_json(u.*) as userinfo,
row_number() over (partition by u.address order by a.date desc) as ucount
FROM artworks a INNER JOIN users u ON a.address = u.address
WHERE a.flag != ($1) OR a.flag IS NULL
) t
WHERE ucount <= ($2)
ORDER BY date DESC
LIMIT ${artPerCall} OFFSET ${(start-1) * artPerCall}`
pool.query(query, ["ILLEGAL", artPerUser])
.then(users => {
if (users) {
res.json(users.rows);
}
})
.catch(err => {
next(err);
})
Called through an API with the following path /artworks/paginate/1/20 where (/artworks/paginate/{page}/20)
The expected result is to get 20 results per call with a maximum of 2 entries per user.
The current result:
It seem that it return only 2 entries per user as expected but once it return 2 for a user on a page then no more result for the same user in the following pages even if they have entries.
Any idea what i'm missing?

Answer 1

It seem that it return only 2 entries per user as expected but once it return 2 for a user on a page then no more result for the same user in the following pages even if they have entries.
Correct, this is what the query does. It selects:
row_number() over (partition by u.address order by a.date desc) as ucount
...
WHERE ucount <= ($2)
If parameter $2 is set to 2 as it is in your example code, then it will select 2 entries per user, not more, before sorting and pagination. If the user has more entries they will be filtered out.
If you remove "WHERE ucount <= ($2)" then you'll simply get all the results ordered by date, but that doesn't sound like what you want.
However, what I think you want to achieve sounds a bit complicated. I'm not sure either it would be great for usability as the results would look quite random to the user. So you will need to describe exactly what you want, with example data.
For example, if you want to avoid one user posting a lot of items with the same date pushing all the other users down in the search results, limiting the number of results per user is a good idea, but perhaps a button "more from this user..." would be a better choice than pushing the users' items down to the next pages.
Suppose you have only two users, user1 posts 20 items with date "today" and user2 posted 10 items yesterday. Do you want 2 items from user1, then 2 items from user2, then the 18 remaining items from user1, then the 8 remaining items from user? Or will they be interleaved with each other somewhat, which will make the date order a bit random in the results?
EDIT
Here's a proposal:
SELECT * from
(
SELECT *,
row_number() over (partition by user_id order by date desc) as ucount
FROM artworks
) t
ORDER BY (ucount/3)::INTEGER ASC, date DESC
LIMIT 20 OFFSET 0;
"(ucount/3)::INTEGER" is 0 for the first two artworks of each user, then 1 for the next 3, then 2 for the next 3, etc. So the most recent 2 artworks of each user end up first, then the most recent 3 artworks of each user, etc.
Another one:
ORDER BY ucount<3 ASC, date DESC
This will put the most recent 2 artworks of each user first, then the rest is simply sorted by date.