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

One alternative is to use a LATERAL JOIN which is a Postgres 9.3+ specific feature which can be described as something like a SQL foreach loop.

class Employee < ApplicationRecord
  has_many :employments
  def self.in_active_employment
    lat_query = Employment.select(:status)
                      .where('employee_id = employees.id') # lateral reference
                      .order(created_at: :desc)
                      .limit(1)
    joins("JOIN LATERAL(#{lat_query.to_sql}) ce ON true")
      .where(ce: { status: 'active' })
  end
end

This fetches the latest row from employments and then uses this in the WHERE clause to filter the rows from employees.

SELECT "employees".* FROM "employees" 
JOIN LATERAL(
  SELECT "employments"."status" 
  FROM "employments" 
  WHERE (employee_id = employees.id) 
  ORDER BY "employments"."created_at" DESC 
  LIMIT 1
) ce  ON true 
WHERE "ce"."status" = $1 LIMIT $2 

This is going to be extremely fast in comparison to a WHERE id IN subquery if the data set is large. Of course the cost is limited portability.