Using Nested Selects for Performance in Rails

It would be slow on MySQL, though.<p>If you EXPLAIN that query, it&#x27;s a DEPENDENT SUBQUERY which has to fetch N*M records. It would be mitigated if the columns are property indexed, but two separate queries (N+M) just as eager loading would be faster and more predictable than nested scans in terms of IOPS.

&quot;But there is no easy way to do this in ActiveRecord without sacrificing eager loading.&quot;<p>Define a relationship:<p>has_many :approved_comments, class_name: &quot;Comments&quot;, condtions: {approved: true}<p>Then eager load approved_comments.

Author could use counter cache to solve the problem.