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

The simplest solution (based on code complexity) I can think of is first fetching the employment ids with their maximum values, then compsing a new query with the result.

attributes = %i[employee_id created_at]
employments = Employment.group(:employee_id).maximum(:created_at)
              .map { |values| Employee.where(attributes.zip(values).to_h) }
              .reduce(Employment.none, :or)
              .where(status: :inactive)

employees = Employee.where(id: employments.select(:employee_id))

This should produce the following SQL:

SELECT employments.employee_id, MAX(employments.created_at)
FROM employments
GROUP BY employments.employee_id

With the result the following query is build:

SELECT employees.*
FROM employees
WHERE employees.id IN (
  SELECT employments.employee_id 
  FROM employments
  WHERE (
    employments.employee_id = ? AND employments.created_at = ?
    OR employments.employee_id = ? AND employments.created_at = ?
    OR employments.employee_id = ? AND employments.created_at = ?
    -- ...
  ) AND employments.status = 'inactive'
)

The above method doesn't hold up well for large amounts of records, since the query grows for each additional employee. It becomes a lot easier when we can assume the higher id is made last. In that scenario the following would do the trick:

employment_ids = Employment.select(Employment.arel_table[:id].maxiumum).group(:employee_id)
employee_ids = Employment.select(:employee_id).where(id: employment_ids, status: :inactive)
employees = Employee.where(id: employee_ids)

This should produce a single query when employees is loaded.

SELECT employees.*
FROM employees
WHERE employees.id IN (
  SELECT employments.employee_id 
  FROM employments
  WHERE employments.id IN (
    SELECT MAX(employments.id)
    FROM employments
    GROUP BY employments.employee_id
  ) AND employments.status = 'inactive'
)

This solution works a lot better with larger datasets but you might want to look into the answer of max for better lookup performance.