Postgresql order by column that not in group

Question

I need to get movies with fresh series first.

In rails I have:

l, o = 30, 0
Movie.joins(:series).order("series.created_at DESC").group("movies.id").limit(l).offset(o)

It translate to:

SELECT  "movies".* 
FROM "movies" INNER JOIN "series" ON "series"."movie_id" = "movies"."id"
GROUP BY movies.id
ORDER BY series.created_at DESC
LIMIT 30 OFFSET 0

But I have error message

ActiveRecord::StatementInvalid: PG::GroupingError: ERROR:
column "series.created_at" must appear in the GROUP BY clause or be used
in an aggregate function
LINE 1: ..._id" = "movies"."id" GROUP BY movies.id  ORDER BY series.cre...
                                                             ^

And OFFSET working very strange, sometimes he duplicate results.

How can I fix this without using sql? Would it work in MySQL?

In Pg I have many little problems like this, how do u think if I change it to MySQL would it work better?

PS: In sqlite it works file

lx00st · Accepted Answer · 2015-04-22 11:54:56Z

3

Use agg max if you want fresh one Movie.joins(:series).order("max(series.created_at) DESC").group("movies.id").limit(l).offset(o)

Without aggregation this query does not know what series to order - it s not a pgsql or mysql issue.

In posgres you have more possibilities for aggregations

edited Apr 22, 2015 at 11:54

answered Apr 22, 2015 at 11:47

lx00st

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