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

Since the title includes ARel. The following should work for your example:

employees = Employee.arel_table
employments = Employment.arel_table
max_employments = Arel::Table.new('max_employments')
e2 = employments.project(
      employments['employee_id'], 
      employments['id'].maximum.as('max_id')
     ).group(employments['employee_id'])
me_alias = Arel::Nodes::As.new(e2,max_employments)

res = employees.project(Arel.star)
      .join(me_alias).on(max_employments['employee_id'].eq(employees['id'])).
      .join(employments).on(employments['id'].eq(max_employments['max_id']))


Employee.joins(*res.join_sources)
  .where(employments: {status: :inactive})

This should result in the following

SELECT employees.* 
FROM employees 
INNER JOIN (
    SELECT 
       employments.employee_id, 
       MAX(employments.id) AS max_id 
    FROM employments 
    GROUP BY employments.employee_id
    ) AS max_employments ON max_employments.employee_id = employees.id 
INNER JOIN employments ON employments.id = max_employments.max_id
WHERE 
  employments.status = 'inactive'