How do I build a query in Ruby on Rails that joins on the max of a has_many relation only and includes a select filter on that relation?

Question

I\'m struggling how to have Ruby on Rails do this query right... in short: to join on a has_many relation but only via the most recent record in that r

Answer 1

After fiddling for a while (and trying all these suggestions you all came up with, plus some others), I came up with this. It works, but maybe isn't the most elegant.

inner_query = Employment.select('distinct on(employee_id) *').order('employee_id').order('created_at DESC')
employee_ids = Employee.from("(#{inner_query.to_sql}) as unique_employments").select("unique_employments.employee_id").where("unique_employments.status='inactive'")
employees = Employee.where(id: employee_ids)

The inner query returns a collection of unique employments... the latest for each employee. Then based on that I pull the employee IDs that match the status. And last, find those employee records from the IDs

I don't love it, but it's understandable and does work.

I really appreciate all the input.

One big take-away for me (and anyone else who lands across this same/similar problem): max's answer helped me realize the struggle I was having with this code is a "smell" that the data isn't modeled in an ideal way. Per max's suggestion, if the Employee table has a reference to the latest Employment, and that's kept up-to-date and accurate, then this becomes trivially easy and fast.

Food for thought.