Q016warning

group_by needs many

group_by is used with a command other than many.

db/queries/example.sql:1:1: warning[Q016]: sgl.group_by is valid only with the many command; query "GetAuthorBooks" uses one

   |
 1 | -- sgl.query(GetAuthorBooks, one, group_by: authors.id)
   | ^

Printed by sgl from a small PostgreSQL reproduction project.

When it is reported

Grouping collapses several rows into each parent, which needs a query that returns many rows.

Use the many command, or remove group_by.

$ sgl explain Q016

Look for GroupByWrongCommand in Seagull's source to see where it is reported.