Rails: How to get objects with at least one child?

Question

After googling, browsing SO and reading, there doesn't seem to be a Rails-style way to efficiently get only those Parent objects which have at least one Child object (through a has_many :children relation). In plain SQL:

SELECT *   FROM parents  WHERE EXISTS (                SELECT 1                  FROM children                 WHERE parent_id = parents.id) 

The closest I've come is

Parent.all.reject { |parent| parent.children.empty? } 

(based on another answer), but it's really inefficient because it runs a separate query for each Parent.

Answer 1

Parent.joins(:children).uniq.all