Two tables:

Categories
  :id,
  :name
  
Instances
  :id,
  :category_id,
  :another_id,
  :quantity

Category.
  joins('LEFT JOIN "instances" ON "instances"."category_id" = "categories"."id"').
  select("SUM(instances.quantity) as quantity, ").
  group("instances.category_id")
  
But I need to filter by :another_id and I have no idea how to without removing some categories without any instances with that id