How to avoid applying the "limit:" to associated schemas?

Phoenix Forum Questions / Help
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1

I’m trying to list some users with links to their posts (per specific topic), and have some basic pagination in place.

Users have many Posts preloaded like so:

query1 = from User, 
    offset: ^((page - 1) * per_page),
    limit: ^per_page    

query2 = from u in query1, 
      distinct: true,
      join: p in assoc(u, :posts),
      on: p.poster_id == u.id,
      where: p.topic_id in subquery(query_topic_ids())
      group_by: [u.id, p.id],
      preload: [posts: p]

Posts also belong to Topics (used here to load only relevant users)

Expected

When supplied a per_page: 1 as parameter the Repo.all(query2) should return 1 User with all associated Posts.

Actual

I get 1 User with only 1 associated post (although there are more posts).

So, how do I restrict the scope of the limit: operator to the parent schema (User) only?
How do I otherwise load and paginate only the users who posted under specific topic and also load such posts per user.

PS: Couldn’t find relevant info in the docs and on this forum or SO, and will appreciate any hints and pointers!

2

This is how sql works. Limit is based on the returned rows, not any kind of table. The only way to do what you‘re asking is loading posts in an extra query instead of joining them.

1 Like
3

Thanks! Can you suggest how to efficiently associate the Posts loaded in a separate query with the relevant Users?

Is there a way to do this with Ecto?

4

Window functions can also make this possible, although it isn’t particularly easy.

5

If you’re looking to preload the posts for the user, then you’re best off by saying preload: [:posts] instead of by joining with a limit.

The reason for this is that when you do a join in sql, you’re going to get 1 row with the left side and right side together, per match. If you apply a limit, it’s going to apply to the final match joined rows and you’ll get 1 result back.

If your association definition isn’t enough to do the preload, you’ll get extra rows back that you didn’t intend for. You can provide a query to the preload and apply your additional joins criteria. Examples of that are in the preload documentation.

3 Likes
6

Thanks, everyone!
Got it fixed using separate queries and avoiding the :join all together. )